He's probably the
kind of man with a 'vulture mindset' - doesn't want to get
downright dirty insofar as to break her present relationship
directly, preferring to wait for her relationship to crumble
(equipped with the typical thinking that most BGR 'wouldn't last'
type of perception).
Now that he realised he has lost the bet, he became desperate. And
being desperate means he's willing to risk his foot by wedging it
in between the 'closing door'... before she gets married.
His idea of a part time boyfriend suggested to me that very likely,
he's keen on a non-committed sexual relationship, because even if
he's emotionally depended on her, usually, that doesn't spark off
the idea of a part time boyfriend in that context. A close
friendship (platonic) would do just as well.
Why would her close guy friend make such a request? I supposed
within their dubious friendship... she has probably suggested to
him something, in which he has gotten the idea that your gal friend
is probably into such sub rosa relationship.
I think the answer is pretty straightforward - if she is even
having difficulty making a decision, I think she is not ready for
marriage... emotionally. 
Cheers
