I wouldn't imagine that breaking hearts or loving someone of a different alignment would make a paladin fall. It's how they go about their actions.
As far as how to play it out, it will depend on the Paladin's personality, and what sort of paladin is roleplayed. A self-flagellating nut job, obsessed with purity? a Dudley Do-Right? Oblivious or not? Or just a good person trying to do whats right in a fallen world? The PC is the part you don't have control over, if you know how the PC ticks (and share it with us), it becomes easier to see how to handle it.
Also to keep in mind: What are your long range goals? How much table time do you want to devote to this before the other PCs get bothered?
That being said, I suggest doing full Tolstoy.
http://en.wikipedia.org/wiki/Anna_Karenina#Plot_summary