I see three possible things here...

1.) J is pimping K out for money they need (both agreed to this)

2.) J is trying to keep K interested in every thing and body so she doesn't think of you, hence leave J

3.) K is using sex to numb her true feelings and J is helping her

Sorry - this is happening to you, but if K is 24 years old then she is old enough to learn not to let others (not even that of a best friend) get in her way of true love... thats when she actually finds it.