What would you do or think if you were in a restaurant and saw a man punch or slap his wife/partner, if she did not seem to mind it?

I went to dinner before with a couple that I know. They are into BDSM, and the wife is the “slave” of the husband. During dinner, he punched her hard on the arm once, and later on he slapped her twice on her shoulder. I was somewhat embarassed because I felt like other people would find that behavior alarming, as I did. I feel like it’s fine to do what pleases you in your own home, or amongst your friends, but in public I think it’s inappropriate.

What would you do or think if you were in a restaurant and saw slap or punch his wife/partner, if she did not seem to mind? I am referring to a couple that you did not know personally, you’ve never seen them before, they’re strangers.