He may well have thought it was a joke. I got a lot of notes like that in my adolescence, and I took them for what they were: stupid pranks trying to get a rise out of me so I could be made fun of. A card saying “I want to jump your bones” may have come off as too blatant to be true.
She would have been better off asking him out for coffee or something, then whispering an indecent proposal in his ear.
As for what guys will do or don’t do, some have higher standards than others. One standard most men adopt pretty quickly is “no crazy” (though this may be ignored by some fellows in dire straits). I knew a guy in grad school who quite literally had three women (that I knew of) throwing themselves at him. The circumstances in each case were so weird that he backed off in a big hurry. (That, and he may have been really disciplined about relationships.) Your friend may well have tripped the crazy circuit breaker with her card–that would come off pretty odd to me.