Let's assume you recommended the best wallet for him with the said amount of 1 bitcoin to be stored in the wallet because we always have to consider best securities to keep out funds safe first. Perhaps that's better for him of he's to undergo a long term investment.



The misconception here's that that phrase "don't put your egg in one basket" signifies that " you should not channel all your expectations such as source of income in just one place". Which is where the need to diversify your incomes was originated from.
The crypto market is highily volatile so holding such amount of funds in just one source with no other sources can be weird when the market condition hasn't been encouraging for a long time because you might be at need of funds and you selling at the odd time might be at lost to you but when you diversify, you can, if bitcoin market doesn't seem favourable in the moment, some other potential coins you invested on might be of good posture to you and while you splits your assets, you're giving you're expatiating your Incomes to make profits from different sources.
You can't look up to other security lines when you've found an authentic one to say you want to diversify.
Talking about splitting your bitcoin assets of such amount to different wallets doesn't really call on security enhancement but your investment plans which you might want to invest on short and long term goal whereas you don't want to temper on your long term assets when you want to sell your short term assets.

Don't put all your eggs in one basket is the best suggestion. But that is just an option because if you think you are okay with one hardware wallet, that will be fine for you. But other people may not have the same thing as you and separate their wallets because of the security.

What he thinks can be acceptable because he thinks about how if I can not access my wallet while all of my assets are in one wallet. But if I have more wallets, I can still access my assets while I can figure out how to access my previous wallet.

It is about convenience for each user about how they will save their assets. We could not have the same method to save our assets because we have different experiences and we learn from the different lessons and sources. But the purpose will have the same goal which is save our assets and take care of them.

You should not think about this twice. Having your coins on two or three cold wallets is better than having it on just a single wallet. To avoid backup complications, I have just one seed phrase with different passphrase for each wallet which I think is good than just having one wallet. So the backup will not be hard for me.

Although, if you have good security like 2-of-3 multisig with 2 online wallets and 1 hardware wallet, I think it is also good. But you will make sure that you avoid malware on the online wallets. Or you can make it 2 hardware wallets.

So it depends but not having your coin in one basket is better.

Let's us look that proverb to be coming from inexperienced users just as Op has made known earlier because splitting you bitcoin to different wallets might demands more tasks to ensure safety to where the seed phrases are being stored.
In the other hand, splitting your bitcoin will help to condone alternative reliance In the case of loosing your funds to scammers on wallet attacks so, you can always have other options of not loosing it all of paraventurely it happens as mistake could be paramount but that is not an excuse to be welcomed.

OP it is best to store your Bitcoin in one place so that you can give it the maximum-security measures and take care of it properly. What you can do is to provide several securities instead of relying on the hardware for security. Set up your withdrawal security to request your PIN, two factor authenticator on the device even before accessing the wallet.

If he still thinks the security measures is not enough a maximum of two wallet is enough. Anything above two wallets he might be jeopardizing his asset.

If we go by the exact meaning of this parable which simply implies:
source
Fron the definition I don't think storing the full BTC into one hard wallet will pose any threat to the asset now or in the future. Here the effort and concentration mentioned in the quote was pointing towards a centralised way of doing things but in the case of the wallet the hard wallet give him full control of his asset, except there is something fishy that you are not telling us

Why split it into two hardware wallets when one wallet can just do the task and save him from having to keep two different private keys or seed phrases. To me I believe it's much easier to save one seedphrase and properly account for it's safety than have more than one.

Make use of a non custodial wallet, try to avoid having all your crypto asset on single wallet, also try as much as possible to ensure that you secure your preferred wallet from the visibility of any other person, I will also go by the idea @franky1 suggested, that you can have a little portion of your portfolio on hot wallet and let your family have access to it in other to distract then away from the main one you're holding, serving as your major portfolio.

Typically the ones you are holding on those exchanges and the ones you consider to spend in no time and the one on wallet 1 and  are for long time holding which, if their is any need to spend anything you have that covered with your funds in exchange 1 and 2 and reduce the risk of signing transaction from the wallet which holds large number each time you want to make a little sales.

If a Bitcoin holder cannot handle one wallet securely how else do you think he can handle two or more wallets. The problem is on the individual, if he can secure his wallet well then there is no need to have another wallet. It is possible that he can lose both wallets at the same time if he does not prioritize security.

In the case of using his Bitcoin for several purposes an investor can have wallet for savings in Bitcoin why the other can serve for daily transaction. This way one will be accessible frequently while the other only for checking purposes.

It could be and could not also. I only view this on the aspect to how far you can keep your  wallet safe. If you fear of being compromised no matter how you store your seedphrase and not open it to the public, then you can consider splitting them into two different hardware wallets, but that calls for more security measures as you can’t save them in the same place as you fear of it being compromised. If you can put all your bitcoin in one wallet and can secure them, it would be better as I see it as more stress splitting it although a nice idea if done to minimize the risk of losing all funds if compromised.