Sounds very odd to me I'm afraid, though here's a couple of slightly nicer suggestions:
1 - Your obviously have a long and, seemingly, happy marriage. Maybe he thinks that because of this his behaviour is OK, as you are both secure in your relationship and have established trust between you.
2 - Is she having problems? Is he the kind of person that always likes to help people in a difficult time? This doesn't make his behaviour acceptable (IMO) but possibly does explain it?
Either way, I do think you need to intervene here but there are maybe a few ways you can do this in a non-accusatory way:
Firstly, there are myriad examples all over the web of where bad things have happened where a real person is very different from their online persona. Is he being exploited? Suggest this to him.
Secondly, tell him that you're jealous! You don't need to accuse him of trying to have an affair etc. but simply let him know that you are jealous because you love him and don't want to share him with another woman on a deep, emotional level (no need to mention the physical).
Have you asked him if you can read some of their conversations? If he doesn't allow this, then I really would try and put a stop to it.
Good luck.