You were wrong. The boyfriend is always wrong. Something you need to learn if you want to remain the boyfriend.
You should have said it was a cheap shot, and you know that the gf would have taken care of her in a fair fight.
Later, privately, you could perhaps discouraged her from fighting over nothing.
Check out the woman on here who got her faced shoved in custard twice. Lolly or some such name. Pointless abuse, never admitted that she lost twice.