Call and observe. Unless you are in fear for your life or the lives of others, that is about all you can do.

Had you thought about what you would do if he started hitting her while you were observing?

Just a tip; in general, the po-po often assumes that the person on the scene, wearing the least amount of clothing, is the person most guilty. It's not always true, but it is an amazing indicator when you roll into an unknown situation. Next time, ask your wife to bring you a t-shirt while you wait.

Be safe.