It depends on if he's really hurting her...
If they are scuffling about and bruising each others ego more than anything else. (Pulling hair, rare instances of slapping, pushing, etc.) Then I would say to stay out of it, it is her place to take care of it.
On the other hand, if he is truly beating her and causing her grevious injury... then maybe it is your place to call the cops next time a fight breaks out.
I guess it all depends on the severity of the situation. Some couples get a little physical when they argue... and to some extent it is normal. But it is NOT normal for one spouse to beat and injure the other; ESPECIALLY with any sort of frequency.
:dunno: