do you think its understandable?

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Here is the situation. Imagine you have just broken up with your gf/bf for a long time because you feel neglected. But you remained friends with her/him because after all you used to be very close friends that you regarded each other best friends before you became an Item. Then recently you found out that you still love your ex only to find out that your ex is in-love with someone else at work. Let us say they plan to get married, will you attend or not? I am nearing out my wits here because they are both my friends and would like them to remain as friends although they broken up. This is what's eating both of them I think. a) The girl wants the ex to accept the new bf and wanted to attend her wedding forgotting he is btw her ex. b) While the guy on the other hand could not accept this new found love because he feels the new love is the one who may have done something to ruin the relationship because he is the ex gf's supervisor. The guy became lonely in the relationship because the ex gf is always busy, and more often spoke about the supervisor rather than something else. What should I tell both of them? Is it understandable the way teh ex bf behaves?