Agree with above posters. Also, ask your friend what they feel comfortable with.
We did not have our Bridesman match our groomsmen because we felt like it might look like a groomsman wandered over to the wrong side. So we had him wear colors that matched the bridesmaid (which was black so it was pretty simple) and we made him a boutonniere that matched the bouquets. When my MOH was the Best Woman at another friend's wedding they put her in the same dress as the bridesmaids and apparently people thought she had stood on the wrong side I would talk to him and ask what his thoughts are and then come up with something that flows and also makes him feel comfortable.