What in the world was the old lady doing there so late at McDonalds.
Anyway, if I were in a seat there with my wife eating a #3, my #1 concern is my wife and then myself. Call the police, let everyone know police are on their way, and then get my significant other to safety.
Depending on the aggressors and their accomplices reactions to my advancement to try and break up an obviously losing fight, that would determine my level of engagement. I would first go to the men in the background or anyone else sitting on the fence and say "We need to break this up now, help me get the victim some space, why they are fighting doesn't matter right now". This will immediately separate who will be helpful.
The #1 thing you need to do as a samaritan is just make some space for the victim, you have no business retaliating and going all NATO on the other women. You can only engage if you yourself become a victim.
#2, always carry some type of defense on your person, at least pepper spray
EDIT#3, realize that a huge portion of the people who take service jobs like these are doing so because they have no choice. Don't ever #%@ with someone at a fast food joint, they are almost always going to be crazier then you ever will be. You will just be #10 added on the probation case