Friends with an antenatal group - we all got on well and 4 of us and husbands/partners met up regularly with babies all good.
One antenatal friend "Jane" and husband "John" lived close and we met up for walks, drinks and whatnot, nice couple...Jane was pregnant again very quickly with a second baby.
Then DH saw John at the airport with his arm around a lady who he rapidly disentangled himself from and introduced as his "work colleague"..DH wouldn't have even spoken apart from the fact he was standing behind him in the queue and hadn't really clocked him until he turned round and got the shock of his life..
Dh and I discussed, but weren't really sure what to do as she had a 6 month old and another on the way.
Never saw or heard from Jane and John again. God knows what he told her, but utter blank from then on (to the whole antenatal friend group). I did tell the rest of the antenatal group eventually at a later date what had happened but only when it was clear we were never going to hear from Jane and John again!
They are still together and that was 15 years ago.
Chose a Mumsnet antenatal group second time round! Still in touch 13 years later and no dramas!