Originally Posted by goodnightmoon
she felt that she needed someone who could give her themselves fully, but she didn't want to let him go.
If she were to really do the work to wrap her head around poly, she might find she could have both. There are many of us here (and it sounds like your fiance may be joining this particular group) who are poly but have at least one mono partner. However, being mono has to be a partner's choice, not a demand. It's no good saying, "I'm going to be with you AND this other person but I want you to only be with me."
There are also situations where people might date until they find the "right one" and then commit to just that relationship, so she could still date him while she continues dating others. If he's okay with the knowledge that their relationship will likely end when she finds someone to commit to just her, that could work too.
I don't have much else useful to add at the moment, I just wanted to point out some possible scenarios that maybe hadn't been discussed yet.