It's not real common but happens occasionally - probably more so with guys, since often girls make the gymnastics to cheer switch a bit earlier. It would definitely help her at college tryouts to get at least some stunt experience well before tryouts. Sometimes high-level gymnasts struggle with coed stunting at first because they're so used to balancing themselves that they have trouble with just staying tight and letting the guy do the balancing.
Her success at tryouts will also depend a lot on the cheer program. At many programs, the coach will see the tumbling talent and say "I'll take her and teach the rest." At some high-level programs where the tryouts are very competitive, she might get beaten out by others who also tumble well but can already stunt and look comfortable cheering in front of a crowd.
Also, note that many college tryouts are on non-spring mats, so that's something she'd want to prepare for.