For stability, I'd say stick with Gingerbread ROMs. My wife and I both have a G2X, and I while I don't mind dealing with the little things that creep up on ICS ROMs, she just wants the phone to work. I have GB version of Eaglesblood running on her phone, and she has no complaints.
That said, as many have said around this forum, this phone is a strange beast in terms of stability. Even between my wife and I, one ROM works perfectly for me but is incredibly buggy for her, and vice-versa. It's a great phone, but it's just a little touchy. Trial and error will probably be the best way for you to find what works well for them... if they'll let you play on their phone a lot.