It depends on what the story is...
It sounds like you have been digging on this girl for sometime and her partner is maybe aware of that and naturally (if he is) he is not very happy about that (I wouldn't call that a jelousy issue, but a genuine reason to worry).
If you want this girl to the point where you'd love to see her current relationship deteriorate so she can jump into your arms then follow Hayward's advice. Just watch out, promiscious girls who like to change hands will not hesitate to give you the slide when the opportunity for someone better than you presents itself...
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~