She was after all congratulating them both in a one liner email/message. No different to anyone else saying the same in passing and exchanging pleasantries in person I don't suppose.
If he had arranged to meet her at a 'secret' rendezvous without telling Chantelle, then that would be another matter and perhaps then it would be seen as cheating. :)
Ways to avoid domestics: I think in order to save problems/wrong assumptions it is always best to be open and honest with your partner and tell your partner you've received a private message from someone you think he/she may become jealous over if they found out about it. :)