Where do you draw the line about knowing what he is doing ? When is is married ? When he has a full time job ? when he has a part time job ? when he has children ? When he retires ????

If he is paying for his phone (which I don't know if he is or not) then I believe she or any parent has no right to look at private messages on the phone. If mum or dad pays for the phone then yes they do.

But that is in my book where you draw the line.