At this point there isn't much that can be done, what goes in must come out. My son went through this several years ago after visiting a county fair. He had to sample all the homemade hot sauces and really suffered through the night.
The only thing I could do for him is give him plenty of water and cranberry juice to flush out his intestines. The cranberry juice did help with the inflammation. I always turn to cranberry juice anytime I feel the kidneys, bladder or intestines need a protective coating to allow the body to heal.
I'd also suggest applying some petroleum jelly around the rectum after each bowl movement to protect the tissue.