about 5 years back i met a guy online, i was in a relationship and was very clear from the first sentence of our conversation i was looking to make new friends only. He told me he loved me after 3 days, got really clingy and was upset when i mentioned i was already taken (hadn't in initial conversation as i like to keep private stuff private and i was clear i was only looking fro friends).
I called him "hun" like i do with all my friends on his social media status about his job interview when wishing him good luck.
Next i know i've got some girl blowing up my inbox, telling me shes his girlfriend of 4 years and she's rougly 3 months pregnant with his kid, demanding to know why i'm calling him "hun". He was telling me he was single and i knew he was going out at weekends having casual sex with women in bars.
I waited, got his side of the story, and he was so convincing. Said that she was his ex who had only just got in touch to tell him she was pregnant, that they had split 3 months earlier because she'd cheated and the baby could be his or the other guys. I believed him, so i didn't tell her about the girls he was sleeping with.
Turns out, he was a liar, they had been together for 4 years and he'd known for 2 months she was pregnant, and was using the fact he moved over 100 miles away for uni to live the single life while still going back to her every few weekends. By the time i found this out, she was already roughly 6 months so any possibility to terminate was gone.
I contacted her and i told her, because i stopped to think what i would want if our roles were reversed. I'd want to split up, and would have needed time to plan change to be a single parent and also a new birth partner, no way i'd want the lying cheating asshole with me. Of course she blamed me, i was the woman telling her that the future she was dreaming of was over. She had lots of questions, i happily answered them. I don't think she ever 100% believed nothing had happened between me and him, despite us living in different cities and having never met. She gave him another chance, and i deleted him out of my life.
She contacted me about 6 months after her son was born, telling me she hoped i was happy, that i was welcome to her ex, and acting like id stolen him. Turns out he walked out on her and the baby and she assumed it was to be with me, despite me having not spoken to him since the day i told her everything 9 months earlier.
This became a bit of a pattern, her son sadly had a health condition and ended up in hospital and she messaged me telling me to tell him about it, despite the fact i had nothing to do with him.
If you're prepared to tell her everything she feels she needs to ask, wants to know, and let her lash out like you're the bad guy, definitely tell her. I know i couldn't ever sit back and know about someone being cheated on and say nothing, especially when im the person the unfaithful person is trying to go after.