Honestly, the safe alarm is not bad idea  but it’s not the main solution. The hardware wallet is not the money. The seed phrase is. If someone gets your 12/24 words, they can restore your wallet on any device and drain it, even if your hardware wallet is still locked in the safe. So locking the device but keeping the seed somewhere in the house is backwards.
In a crowded family house, the threat isn’t just burglars. It’s family, guests, kids, friends of family, people who know your room and habits. They might not even steal — they might just snoop, take a photo, or copy the seed and you won’t know until it’s gone.

What I’d do:

· Tell no one you hold BTC. Privacy is part of security.
· Use a hardware wallet with a PIN and a strong passphrase. The passphrase is like a 25th word. Store it separately from the seed. If someone finds the seed but not the passphrase, your funds are still safe.
· Never store the seed digitally. No photos, no cloud, no notes app, no email. Metal backup is better if you can.
· Don’t keep the seed and the hardware wallet in the same place. Ideally, keep the seed offsite — bank locker, trusted relative, or split in two hidden locations.
· Bolt the safe down and hide it. A safe beside your bed where everyone walks past is the first place people will look. Alarm notification is fine, but it only tells you after someone already opened it. It won’t stop them from copying the seed.
· You can use a decoy wallet with a small amount in the obvious safe. Keep the real stash elsewhere.
· If the amount is serious, learn multisig 2-of-3. Keys in different locations. That’s the real answer for a crowded or untrusted environment.
So your friend’s idea isn’t bad, but it’s not enough. The alarm is a nice extra. The real protection is seed secrecy, a passphrase, and not keeping everything in one house. If you can’t secure the seed properly, don’t keep life-changing money in Bitcoin yet.