First of all, after you are around your friend and her little girl, when your son begins the babbling, remind him to use his words so that you can understand him. I believe he is using her babbling because he is trying to learn to communicate better with her. He is still in the stage where they learn by imitating. I wouldn't stop socializing with my friend or her child. I might remind him during the play date to use his words so that his friend will use hers.
I wouldn't be concerned with them spending time together. I don't think he will forget his words. He might even encourage her to learn more words.
However, I would be concerned about my friend's child. I might casually say when we next played with them, "I haven't heard *daughter's name* say any words yet. Does she talk yet?" If the answer is no, I might go on to say, "That is unusual. Have you mentioned it to your doctor?" or "Have you thought about having her tested to make sure her hearing is alright?" It could be a problem with her development (nothing permanent. some kids just need a little extra help to catch up to their counterparts faster so they don't fall behind.) or it could be an actual problem with her hearing. Alternatively, it could just be that she doesn't talk around you and your son. I would think that your friends daughter should at least be using words to identify what she wants by this time (even if she doesn't have a lot of words). It may be nothing. She just may not be ready. But I know from friends' experiences that the earlier the intervention if it is a developmental delay, the easier it is and less time it takes to catch them up. If it is a delay and it waits, it can affect other developmental milestones as well. Like I said, it might be nothing. If she spoke with her doctor about it and he is not concerned, then that would put my mind at ease. If, however, she hasn't asked, it may mean that she doesn't know that it could be an issue or that there could be help out there if there is an issue (oftentimes help with no cost to your friend).
Anyhow, I hope this helps. Good luck. And if you are looking fore moms and socialization, check into the MOMS Club. There are several around Pittsburgh. I belong to MOMS Club of Pittsburgh East. It is a wonderful place to meet new moms, join playgroups, make playdates (sometimes even just ones for mom to come to like our book clubs) and even get information about area happenings.