It would help if you said what browser your Mum is using, but assuming it is IE, the only way I can get it to come up not maximised is to close it either from minimised or a partial window. I've been all over the 'Options' dialogue looking for a setting and can't find one, and I had a look at 'View' as well.
If I'm right, explain to your Mum about the three buttons in the top-right corner and what they mean, and tell her that if she wants to close IE ( and Firefox works the same way ) just to click on that little box that has the 'X' in it. IE will close, and should reopen in full screen the next time she wants to browse the web.
Good luck - doing it by phone is not easy !