I'm all for (safe) friendly sex when both (or all) parties involved are after the same thing and can discuss it with no reservations. All that stuff about how emotionally weak an susceptible women are and how they're dying to have babies is a stereotype, not a universal truth.

But if she's bonkers, man, DON'T DO IT!

It's very possible she won't or can't make a rational choice about sex, and not only are you going to look like the the bad guy if it turns out badly, you WILL be the bad guy. It's not sporting to take advantage of the fruity just to get your kicks, not matter how willing they think they are.

Find a different "friend."