Is 許せ【ゆるせ】 something that's said? I heard from a friend that it is used to mean forgive me, but I find the use of the imperative in such a way to be strange.
It is not strange at all. It appears to me that many Japanese-learners let the translated English words bother them too much. Then again, the same happens when we Japanese learn foreign languages as well.
許せ is very often used in informal speech among close friends as a sort of "tough-guy speech". It is heard most often from young male speakers. Using the politer forms can only sound out of place in certain situations, and I really do not think it is a Japan-only phenomenon.