Well, first, I'm really glad there are people like you out there. It's great that you try to help others. I've not had problems yet, but if I did, I know I'd be really grateful for someone with know-how (I have little, though I'm never far from a gas station/place to call a taxi).
However, I don't blame you at for not going out of your way to help him...maybe it's that I'm female and we always have that extra "is this a creep?" cautionary thing going on in our heads, but his whole outfit seems weird to me...and it sounds like you couldn't even see his face.
I definitely err on the side of "no worries" and am not scared of much - certainly no one has ever accused me of being a rule follower and I scoff at a good deal of cautionary advice - but even as a new commuter I can't imagine what people riding without lights and in dark clothes can possibly be thinking...if it had been me and my lights had given out or something, I'd probably have asked if I could follow if I saw someone like you...but there again, maybe guys have a harder time asking each other for help? :-)