Well if he/she is just meeting up just to form new friendships.
I think it's fine, well your bf/gf is guilty what they did behind
their backs they will eventually tell them. The more you pester
them to find out the truth, they will find you annoying leading to
a breakup.
And I am certain that you don't want a relationship to end that
way, just let it go and your bf/gf will tell you. If she hasn't
come around, just talk to her about it and see what she says.
Tell her how you feel but don't make it obvious that your
jealous.