This is a bit of a long story but will try to make it as brief as possible.   


I dated a guy for 9 months back in 2016. He was from a town 3hrs away and so we didn't see each other through the week, only on weekends when he came to stay with me (I was a lowly student, he had a job and could afford the weekly train fare). We talked non-stop, texts, photos and video chats. So it was heartbreaking for me when he suddenly stopped responding for 5 days. He wasn't on fb/insta but I remembered him mentioning twitter competitions. Queue super sleuth mode.  
My super desperate self trawled through every account under his name until I found him...he was alive and tweeting since he went AWOL. Trawled through his tweets and I found one mentioning 'his gf' with another user @'d... she had her insta linked to her twitter and so I fell face first into this strangers wonderful shiny documented 3yr relationship with my bf. It was absolutely horrifying, I cried for days.   


My friends talked me out of contacting this girl, I sent him one last, long, explosive message and 'moved on'. It's coming up to 3yrs now and I have the most wonderful loving partner, I am genuinely happy with my life. But I still get this sick feeling when ex's come up in conversation, that I never tried to warn this poor girl. It came up again last night and I caved and looked at her insta this morning (we share the same first name so it's a name I'm not likely to forget) - they're still together, bought a house together, a dog, engaged, celebrating 6yrs together.   


WIBTA for telling her? I just need to know if I'm doing the right thing by leaving it when my heart sometimes says I'm not.  The level of sneakiness on his part always made me feel like I was not the first. My partner is ambivalent. Thoughts?? 