Can't believe the "mind your own business" people on here.
She deserves to know. I'd be furious if a friend or acquaintance knew and chose to keep quiet.
I have folding money that says they are not in any kind of bloody "open relationship." So many deliberately naive people on this site.
I'd invite her out for a coffee, turn the subject to your respective home lives so that I can get a sense of whether she does in fact know or suspect something already, then break the news.
She might, possibly, refuse to believe, blame me, and get angry and defensive--it does happen. But I'd be happy to take that risk, rather than risk her being strung along for another year or two.
People who say "mind your own business" are basically saying "I'm happy to leave another woman to possibly plan a baby or joint financial decision with a cheating rat and be humiliated, rather than take the tiniest risk that I might feel awkward for half an hour or run a risk of falling out with someone. My own feelings are, after all, the only thing I actually care about or am interested in protecting." It is a shitty form of moral cowardice.