Have your girlfriend inspect the chamois on her bike shorts. A lot of them will have the chamois seam right where the thigh joins the crotch area, and this can definitely lead to irritation there. Perhaps she should switch to shorts that have a chamois with a "wing" section that extends outward from the center to cover this area where the thigh and crotch connect. Looking through my own shorts, the ones that have this extra chamois piece include the Pearl Izumi, some of the Terry models, and the Shebeest shorts with the "She-sensitive" chamois - I am sure there are others as well. It's just a small extra piece of chamois that extends out from the main portion sort of like a "wing" - and believe me, that tiny little extra cushion can really help with a situation like what you describe is occurring with your girlfriend.

I hope all this helps. Please do let me know how things work out.