And then…you are going to have to deal with your anger. You let him go before — now, you really need to let him go. If you cared for him before, and you think he’s a good guy, and if you love your friend, and you think she’s a good person, then you should be happy for them. It will be hard. I can almost guarantee this. It will be extremely hard. You may not want to be around them at first…it may take awhile before you’ll want to be around them at all. It may take even longer for you to be around them with them being all mushy lovey-dovey…ick…but if this is the kind of friend that you want standing up with you when you’re getting married as your bridesmaid, or vice versa, or whatever, then eventually, eventually, you will have to tell her that you are happy for her. And mean it.
It would make sense that these friends of yours (ex-boyfriend and friend, whatever), who recognize and value your wonderful qualities, would have the same appreciation for the traits that they recognize in each other. (I would suggest the movie “Catch & Release” for this concept…). Now, if these two are planning on a fling or something short-term and have no long-term expectations, I could see why it would be rather inappropriate. However, if this thing between them has long-term potential, you’re gonna have to learn to stick it out, and be civil!!