If I convinced my bf/gf to leave their spouse, will they ever return back?

Probably not, when a Sims leaves another Sims they get pretty upset. 

@Acidus150

If you mean will they still be in the game. Then yes they will but as @Phantomlover1717 said their relationship will have taken a severe dib. It is unlikely that they will recover enough friendship to get to be girlfriend/boyfriend again even if you decide to break off your relationship with the sim.

I did this with Eliza and Bob, It makes the sim who was convinced, happy to take a break from his/her spouse and the spouse VERY upset. They fall out of contact aswell so no, they won't ever see each other again. They'll be no more...🤔

I had this happen in my Sims 4 game. If your Sim is married as my Sims were they become divorced. Can they get back together Yes, they are back together and now have 2 daughters. you just have to try if you wish as you would in reality. If you go visit the cheating spouse its a choice one is Se-duce. that worked.🙂

It's all up to you. I think their traits would matter. Both were nice as far as their personality.

Hope this helps, I am not an EA employee I am just trying to help. So if it did help please grant XP points ty.