When are opposite sex friendships crossing the line?

So, my husband has female friends from high school. I am ok with that because I also have male friends. Lately, he has been talking/texting one friend in particular who is going through a bad break up. We'll my husband took it to the next step and went to her home just the two of them. I feel like it was inappropriate and anything could happen from him "consoling" her. He thinks I am over reacting. I just wanted to get other opinions on the issue. Does anyone this this should not happen again or should I just got with it?