What I do is:
Warn the commenter that his answer is being stolen
If the commenter cares and answers the question accept that answer and vote it up. If he does not: stop caring too and accept the stolen one.
If the original posts an answer, leave a nice comment below the stolen answer like this:
It is considered rude by our community to steal someone else's comment and convert that to an answer... To remove the negative reputation points, [edit] your answer, give a different solution, and drop a note so that I can review my assessment of your answer. Alternatively, [delete] it altogether and the negative reputation points will be reversed automatically by the system. (I do this too for answers that give me negative reputation points) **;-)**
#And then downvote the stolen answer!