She probably needs a bath first. Then you can use Chlorhehadine diluted with water, or Vetricyn spray (Pink Eye formula is best because it's the strongest) daily.

When it dries up cover with Nu-Stock or Wound Cote, or whatever topical you like. Sounds like either a hot spot or bed sore from being thin and in housing without bedding.

A good diet always helps also.

S_B