board too thin?

How thick should a longboard for a beginner be? I'm wondering because I ride a 3 and a quarter inch thick board (pretty thick), that is 9'6" and I love it. My girlfriend (also a beginner) has a 9'0" that is a lot less thin, like an inch thinner at least. She is struggling while I have been doing pretty well. I let her try my board yesterday, and she stood up her first time. I used hers and did okay, but had a lot more trouble than I do with my board. Is a 9'0' board that is about 2 and a quarter inch thick too thin. Should she try to sell it and get a thicker one? Thanks for the help. 
