I actually managed to solve it in another way - the issue was Sanders was aggroed on my friend, but not on me. Apparently his permission to enter the attic does not apply party-wide. When my friend used Play Dead the glitch occurred with Sanders repeatedly exiting and entering combat. The regular methods you suggested did not work, couldn't use anything or Escape or whatever, so I was just trying anything at that point.
The solution was to simply de-equip an item to cancel Play Dead, after which combat proceeded more or less as normal (though very slowly).
Thank you for the help!
