As others have said, you can sequester IoT devices to a VLAN that has no internet access. Most of the common devices (lights, switches, sensors) added to smart homes work perfectly fine without access to the internet. Voice assistants are the biggest security/privacy hole since all commercial options are from big tech companies and phone home constantly. If you set up a local homeassistant instance you can get a ton of functionality out of smart devices with no direct connection to the internet. You need to decide how you handle accessing homeassistant from outside your home if that's something you want but there are plenty of options to choose from for that.
One thing I will say that I refuse to add to my home is any kind of smart locks. No matter how much I trust my security setup, I don't trust it with the ability to unlock my doors. If there was one that could only lock them electronically but required being manually unlocked, them maybe. But I haven't seen a lock like that out there.
That's great that they help your friend like that! As someone that doesn't face any kind of accessibility issues myself, it's easy to overlook those kinds of benefits that these devices can provide. In situations like your friend's, I'd agree that any potential security cons are outweighed by the pros (especially if the alternative before was having to leave the doors unlocked anyways).
Agree on the convenience of voice assistants. I've got various models of Google homes in my house that I use for voice controls on anything I don't have a good way to truly automate. Different people will have different tolerances for how okay they are with the data things like that can gather. One day I might try to set up one of the local network voice assistants but those can take a lot of work to get just right. Always a tradeoff of convenience and privacy.