Defending someone after they leave the room is good. Doing it while they’re in it is better.

Both versions are decent and they cost different amounts. Speaking up once somebody has gone is safe: the atmosphere has moved on and nobody has to back down in front of them. Doing it while they are sitting there means the room has to deal with you, and it means the person hears it.

The next time it happens, say the thing in the present tense, plainly, and then change the subject so nobody has to perform a climbdown. One sentence is enough. Look at the person afterwards so they know who it was for.