There were 3 options. Lol.

It sounds fairly innocuous, and since she has been a friend since before you were dating, I would be careful before making any assumptions.,
He hasn't given you any reason to disbelieve him.
What makes you so certain there is something to it?

Maybe wait and see, if he continues being open and honest about their contact, you could offer your own take on her problems.(to him of course.)
If he becomes secretive, which will also happen if he feels wrongly accused, maybe just ask how she is doing etc.

Ultimately if he is going to cheat nothing you can d will stop him, and being wary of any woman who crosses his path, perhaps says more about you and him, than him and them.

Good luck, and if things are bothering you, voice your worries, calmly and rationally. Give him the opportunity to reassure you.
Try and remember,her motives aren't important. His are.