Getting a cramp while swimming is the worst that could happen to us people. I think the best answer here is to relax, float and massage the cramped muscle because, if you try to keep swimming the muscle could tighten more and could get out of hand and could cause you to drown.
Maybe test her theory of being allergic to cats or maybe even some of the scents or something from her friends house. She should also consult a doctor to be sure.