When is she having pain? When she's doing the kick, or later on after class? Is it more muscle pain, or deep in the joint?
If she's having pain doing the kick, or some muscle soreness after, then some of the earlier comments about stretching, muscle strength etc are probably right. She may just be overdoing it and trying kick harder, higher, etc than she has the strength or flexibility for.
If she's having pain deep in the hip joint after the kicking, I'd be a bit more concerned. About a decade ago, I tore the cartilage in my hip on one side. After that, when I would do any turning kicks (side, roundhouse, hook), I would later on have a great deal of pain, like someone was sticking an ice pick through my hip joint. I ended up needing to get my hip cartilage & joint capsule surgically repaired, which meant no exercising for like 8 months and lots of PT. It's fine now, other than some occasional achiness.