Okay to avoid drip feed here is the background context:
Me and guy, call him Dan, met in college and had a thing together. I found out he had a gf at the time and demanded he told her he was playing us both otherwise I would. Apparently he "did it", said I ruined his relationship asked if I was happy with myself blah blah. Move on to sometime later he broke up with her started pestering me, I fell for it out of curiosity and we became a fwbs. This went on for years after college until he eventually got a proper girlfriend, call her Sarah. Me and Dan ended on bad terms and I cut him out from my life.
A couple of years later Dan popped back up on the scene and started talking to me briefly and asked me if I wanted to come down to his university house even though he was still with Sarah. I was fuming that so much time had passed and he was yet still the same sleazy guy, he quickly tried to retract his statement and we stopped talking.
Which brings us to the present - so throughout the past two years Dan has been adamant on trying to get back into my life. Sending friend requests on everything, pretending to be someone else through a friends social media account to talk to me, ringing on private number. Eventually very recently I gave in as I wanted to know what he was being so persistent about. We got to talking and he tried to apologise for the way things went down in the past. I had planned to just hear him out then block him but surprisingly we both have really nice conversations and to be honest I enjoy it. He is still with sarah. Both me and sarah have a mutual best friend Emily. So here is the thing. Me and Dan have been talking and he has been asking lots of do you think we could of been a thing, did you ever feel something questions. Which I guess is fine as I did ask him if he ever liked me out of genuine curiosity. But he is also very flirty, lots of I know you are still crushing on me ect comments. I let it go over my head and dont entertain it. I asked him if Sarah knew we were chatting and he said he would tell her. He still hasnt. He has now suddenly changed his tune and said he doesnt want to tell her incase she gets the wrong end of the stick, but he will eventually. In the meantime he has asked (the cheek of him!) That I keep it a secret. The deja vu is real.
I wonder if and keep going on back on forth as to whether our conversations are inappropriate or not. I think the thing that makes me feel like they are is because A.) He hasnt told his gf and B.) He has moved far away and is in a ldr with her and constantly goes on about how lonely he is and how he is doubting things. But that he is happily in love with her and they are going strong (confusing much)
So my issue is I'm due to see Emily this weekend and I'm in two minds as to whether I should tell her or not. Part of me thinks no because I dont want to be involved in any drama, ther other reason is I could tell her and nothing could come of it. I have a strong feeling sarah knows what Dan is like and will do nothing about it so all it will do is hurt her. The other part of me thinks I should because obviously this is abit wrong (the not so subtle flirting) and I dont owe him anything to be keeping secrets.
What do you think I should do? Sorry for the long read but thanks if you have got this far!