What part of her arm? Forearm? Elbow area? Shoulder?
It's pretty tough to prescribe a course of treatment without first knowing more details. Is this common with her, or is this the first time she's complained about it? Joint pain might indicate something a little more serious - possibly caused by something that needs corrected in her mechanics. "Tightness" in her forearm could simply be fatigue, or maybe she's just learning to throw a different pitch, using muscles that she hasn't used a lot.
Make sure she's not throwing with just her arm - the legs are just as important, especially as she gets older. Make sure her pitching coach is aware of any pain she is having. I would also suggest using a doctor that has a specialty of sports medicine. My DD never iced until she was in college, and only because of sore forearm muscles from spins. But every kid is built different. Do exactly what an experienced sports trainer or physician recommends, and you can't go wrong.
Conditioning and proper form are ultimately the keys. A good indicator of a pitcher being in excellent condition AND sound mechanics is that her legs will give out before anything else.
