I'm a guy and although I've been very horny at times, I can't say I've ever actually had blue balls. Personally I think it's a joke & a lame excuse to ask a woman for some form of sex. You should have handed him a bottle of lotion and a box of tissues and said...here you go.
Speaking as if this was me and my partner, uh, if he says I HAVE to help him, fuck no, I'm not giving into some sort of pressure just because he's horny, I'd never force him either. If he just ASKS for "help" of some sort nicely, if I'm up for it, sure! I just can't stand that sexual "pressure" when people say you HAVE to do something whether you're in the mood or not, that's not ok, regardless of how horny or pent up the other person is.
if it's a STRANGER? Man I'd about punch him, that's fuckn RUDE. No one EVER "HAS" to help you do shit, if that's your pickup line, gay or straight, man or woman or anyone inbetween, that's fucked up.
If it's said as a pressuring tactic, hell to the NO. I will help out if I want to. Asking if I want in on the action because he can't get himself off is one thing - it's another completely to be like "you have to because BLUE BALLS" I mean come on, seriously. Who does that.
It would depend a lot on the situation and his tone. If we were already starting to get playful and/or he said it jokingly, it'd be fine. Same for asking nicely (and being able to take "no" for an answer if I'd really rather not help).
If it was a deliberate attempt to pressure me, then FUCK OFF SIR! If you're that desperate, there's a remedy: it's called masturbation and it works just fine.
It would depend on the person and context. If it was clearly pressuring HELL NO. I've gotten the female version of blue balls and I'ved lived with it and so should he. If it's all in good jokes maybe I would entertain the idea.
I said "other" because he would never need to give me an excuse to "help him out." If he wants sex, chances are, so do I. Unless I'm sick or in an awful mood, I'll gladly jump in bed with him, and if I'm sick or in an awful mood, he wouldn't ask that of me. He has two hands he can use himself.
If MY guy said he had blue balls, I would know it is a joke and laugh at him. Now guys in the past that have tried that argument with me ended up getting a lesson in anatomy and physiology with a bottle of lotion and tissues to accompany them to the bathroom.
I wouldn't believe him for one minute because I have a higher sex drive than he does.
That said - Hell yeah I'd help him out. But then, we're talking about my Master here, not a guy I was just dating. If some guy I was dating told me that, I'd probably just be annoyed and toss him aside.