Ok, let's say you had a oneitis for years, and you were "such a good friend", then life brought you apart.
Years later she drops in town, calls you, meets you at a party and wants to talk to you, whatever...
You might even be over her, and take it as a chance to see an old friend.
DON'T.
What's happening is she's hitting the wall and none of the chads she preferred to you is anymore availabe for relationship or betabuxxing, so she decided to pick YOU out of her sleeve.
She might need someone to provide for a kid or for her, she might be in debt, she might need a visa or the courage to dump her current husband, it doesn't matter, just remember she doesn't mean well. She gave for granted you will always be available and you'd do anything for her attentions and maybe a little sex.
Wanna pump and dump her? Good luck, if you're cold as ice you'll have a lifetime chance, but don't forget she knows your secrets and how to manipulate you.
If you're 20 something you might think it's odd, or impossible, if you're past 35 you probably know well what I mean.