me and current partner started dating a few months after they broke up with their ex, I know their ex but we're not friends. Said ex still has feelings for partner and I feel I should tell them.

I'm a very direct person and don't plan on making it a therapy session or a discussion of some kind and I'm not sure how that person would react to the news.

My partner doesn't feel obliged to them because they dated several times after and didn't say, which my partner is fine with, whatever they went their separate ways.

What would you recommend?