I wouldn't allow her to just use her point to invoke his aspect. I would, however, allow her to attempt to compel any of his aspects, thus giving him a fate point should he agree, and then forcing a compel on him later. Then he can immediately use that fate point.
In your example, if the person in danger also had, say, Harry's "Epic Wise-Ass" aspect, I'd allow the would-be fate point donator to go "Hey, sissy, you gonna take that hit without even a bad one-liner?" at which point the defender could say something witty and dodge. However, the very next time he goes out, he's getting pulled over by the cops for a broken tail-light, and is going to merrily role-play mouthing off to the officer, possibly getting arrested, and not getting any fate points for him (because he spent it on the roll).
I like this better than letting her spend the point for him because it causes more mayhem.
Alternatively, if she had an aspect of her own, like "Friend of so-and-so", she could potentially get away with invoking her own aspect to give him a bonus... if she had the action available to do so. This represents her pushing him aside, grabbing him at the last minute, ect.