Sac-
You don’t own the dating rights to the person just because you dated them and would still have dated them if the guy didn’t say no (which is obviously the case)
Your friend has the right to go out with whoever she wants
the guy has the right to date any girl he wants, including your friends.
If they went out and didnt get engaged, then obviously there is no reason to harbor any bad feelings
and if they ended up getting married that just proves that it was meant for her and not for you