Actually if they talk as friends is OK. Unless he is like flirting. But I guess he is flirting cos he had to do it secretly. If he wanna be friends with his ex the least he could do is to let his gf know. Sometimes some guys just dunno how to take the initiative.
For me I normally talks to some of my exs because I think its better to be friends than enemies & its solely only as friends.
You should console your friend like tell her to talk to her bf. Cos trust is very important in every relationship