Is it legal for a 19 year old to have sexual conversations with a 16 year old?

well my girl friend's sister is 16 and she started dating a guy when she was 15 and just today we found out he had been having sex chats and web cam conversations where both of them are naked... disturbing yes, im sorry for that. but all aside since she started the relationship when she was 15 and he is legally an adult, is this breaking sex offender laws? (she lives in iowa, he lives in arizona)