Ladies, ladies, please stop stirring the cauldron....
Take yourselves all back a step. Stop trying to see smoke when there is none.
You are applying female logic to a male... always dangerous.
Men can be pretty pathetic creatures, this is ALL and I say again ALL about EGO.
The unhinged woman (and she IS unhinged) is calling him and calling him, and a small part of him is flattered. Deluded that this means she really cares for him.
She doesn't, she thinks she does, thinks he is The One, but the truth is actually that she is obsessed, for whatever reason, it absolutely matters not why, but she is. HE, however is flattered in some small, possibly unconscious, way.
As OP has said again and again he answers her calls 1/10 times, out of politeness. You'd all be right in shrieking He's a player if he were taking all her calls, but he isn't. OK this 1/10 IS kind of keeping her on the hook somehow, bit many of us hate confrontation and to tell someone off that is calling up and being nice and interested in you is mighty tough to be hardfaced and rude to.
Factor in if you are in any way insecure ... which many, many men ARE.... And there you have it, the hanger on and the bemused bloke.... Men!, They are very simple, simple creatures...
OP, you can trust him, don't worry,. he's talking to you about it, you know what is going on.
As far as resolving this, he certainly shouldn't call her, not even to tell her not to contact him, she will interpret this as a sign that he doesn't want her to stop contact.
He needs to perhaps answer one more call and say to her that he's been meaning to talk to her for some time to tell her that is engaged, he is getting married and that really there is not going to be a chance for her and him to be together. That he hopes she understands that it's not right for her to keep calling, and it's not something he wants her to do. That he wishes her luck in her life and finding someone else that wants to be with her. He also needs to tell her that he will not take another call from her and will change his numbers if she keeps calling. Politely and calmly, he can get the message across.
Trust me, I've seen all this at close range. Sit him down and tell him that she could become more obsessed and it's best to let her down gently, but firmly.
Now, Ladies, unless any of you on this thread have more than 10 years direct experience of this, I think I can be relied upon to be giving best interpretation of wierdo meets DH/DF.
It's male ego. Pure and simple, stop keep telling the OP off and saying he's cheating. He isn't.