I haven’t heard of that but you might run into problems with Monzo’s fraud engine if you’re logged in, in the UK & your nephew transacts in Asia.
And since you logging into the app will log him out, that might make life difficult if he doesn’t have reliable access to a data connection.
To keep things simple, how about getting him a Monzo to sign up for a Monzo card too (assuming that he’s over 18 years old)? Then whenever he needs a top up, you can send the money from your account - it’s an instant transfer.
Especially as it sounds like you’ll want to keep using your Monzo account while he’s away too 