WIBTA if I reported my ex to HR for saying I ruined his engagement on purpose?

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The original was posted on /r/AmItheAsshole by /u/unluckyashell-ta on 2023-09-23 11:17:51.

(throwaway account)

my ex "Dylan" (25m) and I (25f) met in high school and dated for almost three years. I broke up with him after I discovered that he cheated on me with a girl who had just turned 17 (he was about to turn 20 when it happened) and I went no contact almost immediately.

after graduating college, I moved to a different country to get a job in my field of work, but since he got a similar degree and I have terrible luck, we ended up meeting again a couple months ago when he was hired into the same company AND department I'm in. he tried approaching me, but I told him from the get go that I really didn't want to talk to him more than necessary. he got upset and said that I was being unfair, that he was engaged and just wanted to be friends again, but I held my ground.

our company's anniversary party dinner was two weeks ago and we could invite our partners, but I'm single so I just went by myself. my ex went to the party with his fiancée "Mary" and the seating plan went by departments, so we were at the same table and the atmosphere was a bit awkward. later in the night, when people had had some drinks already, one of our coworkers asked Mary if she knew that Dylan and I had dated before. she said she didn't and asked why we had broken up. since this was a work dinner, I just said that I didn't want to talk about it, but she insisted until someone else at the table intervened and changed the topic.

a bit after I saw my boss leaving, I said my goodbyes and tried to leave too, but Mary followed me and basically cornered me in the parking lot, saying she really wanted to know why Dylan and I had broken up. I said that was between us and it was none of her business, but she got pushy and wouldn't let me leave, so I eventually snapped and confessed that he cheated. she went back inside and I went home.

monday at work, Dylan told me during our break that Mary broke off the engagement and it's my fault for telling her he cheated on me, so I need to convince Mary to take him back. I told him what happened in the parking lot and apologized for telling her bc obviously I shouldn't have, but I refused to help him get his fiancée back because I'm not gonna lie for his benefit. he said I'm being a bitch and just trying to get back at him. some people started throwing some weird looks at me after that and apparenly it's bc he's been badmouthing around the office me and saying that I ruined his relationship on purpose bc I was jealous.

I told my friends I'm considering reporting him to HR for pulling this shit. some agree that I should do it, but some think that risking his job for this would be too much since a bit of office gossip isn't really gonna hurt me and he will let it go eventually. so, WIBTA if I went to HR to report him?