Don't quit because of an asshole like that. Don't give them that power over you. It's a little alarming that your manager entertained this person without chewing them out (or perhaps they did, but not in your presence?) I'd recommend speaking to a manager (preferably a different one that wasn't part of that conversation, assuming there is more than one manager for your location). Let them know that it's upsetting to hear "disparaging comments about my disability" and if they know what's good for them, they will shut it down real quick. I don't exactly know if making fun of a coworker for a disability goes against the ADA (Americans with Disabilities Act, meant to protect people with disabilities from discrimination), but if the manager you speak to has any brains, they will see that an employee making such disparaging remarks about a coworker is an enormous liability. Be ready for the coworker to retaliate, doing small little things that may not *technically* be against the rules, but meant to get under your skin. Report those incidents as well, no matter how small they seem. Best of luck.