I suppose 'most humiliating', is another thing that means different things to different people.
But, personally, the most humiliating thing I would submit to is being ******* by video and/or pictures of sucking her lover's cock, taking his dick in my ass, or even gf fucking me with a strap-on. She could point out to others how hard my dick was while all this was happening.
Closely related, being femmed for such an occasion. The full monte; makeup including eye shadow, blush, and lipstick, having to wear a nighty or stockings and garters, then being turned over to her lover to use as he pleases. And knowing she has the pics or videos to show to anybody she pleases, especially if it is a friend of hers, and I have to interact with them knowing she has showed those pics to them.
Of course, any of these things happening in front of a live audience would carry the same weight...