I have a Leader that was suspended from his duties for domestic violence, upon investigation it was unsbustantiated, now he is receiving a Relief For Cause NCOER, Is this legal?

Unfortunately Yes this can occur!  The references for these actions are:  AR 623-3, DA PAM 623-3, and AR 600-20

Basically the Commander can relieve the Leader because of personal or professional conduct, behavior, characteristics or performance of duty warrant a removal. The perception of such conduct can also be used as a reason.  Basically when a Commander loses confidence in a Leader they can be relieved.  Now there are certain steps that must be followed in most cases.  Usually before a relief action the Soldier must be provided an opportunity to improve.  In cases where the conduct is so severe this 30 day period can be waived but it must usually be waived by the 1^st General Officer in the Chain of Command with General Courts Martial Authority. If the Leader believes this action is being taken unfairly he can request a Commander’s Inquiry and if this fails he can Appeal the NCOER.  AR 623-3 and DA PAM 623-3 describe the process for both actions.  There is also an NCOER appeals guide.  I have attached one for you review.  Here are some extracts of the doctrine that apply to this action.