If you care: Sometimes, just looking may help. Just don't turn a blind eye to it, which is what most men do in such situations.

If the woman feels she has a sympathetic supporter, it is easier to defend herself and if the jerk thinks there is an eyewitness to his assholish behavior, he may tone it down without you needing to say something. I think saying something may be counterproductive (because when accused people are quick to justify, and that makes them LESS likely to back off, not more) but just being aware can sometimes help.