Your guilt shouldn't be a factor here. Don't be selfish and try to make yourself feel less guilty. You should focus on making sure you treat her with the respect she deserves as a human being.
If she approaches you again, be honest with her and tell her that you aren't interested in a romantic relationship. Don't be her friend because you feel sorry for her, you'll just end up resenting her. If you don't want to have lunch with her, then tell her you aren't interested. Be firm but polite. Don't make excuses, but don't give reasons if asked.
It may infuriate her, but your personal feelings about her looks and condition should not be brought up.
R