If anyone has a problem with my 'no problem' response after a doing a him/her favor, that person will have an even bigger problem because it'll probably be the very last favor I do for him/her.
Seriously though, as long as the person speaks from the heart, the words shouldn't matter so much. Why get your britches all bunched, over another person's lack of formality? Are we all so perfect in our speech habits, as to be truly qualified to correct anyone with speech less 'refined' than our own? Even if we are, is it worth hurting the other person's feelings, just for our own inflated egos?