Rodent Ulcer on Cat's lip

Hi Everyone.My Friends Cat who she rescued from the streets about 1 yr old has a rodent ulcer on his lip.She had him neutered,shots& tested for FIV & feLV both negative.Tested positive for hookworms and was dewormed.She has had him strictly indoors away from her other cats to slowly introduce them.I personally have never dealt with rodent ulcers.What I read is on the web allergies possibly??? But then I read about viruses or something so thought I'd ask here.Anyone have any experience dealing with rodent ulcers as per treatment & did anyone fiigure out what the cause was of them or it? Any help would be greatly appreciated.Thanks C.

"An indolent (painless and slow to change) ulcer, or rodent ulcer, is usually found on the margin of the cat's upper lip. The indolent ulcer has a very unique appearance and typically does not require further diagnostics. At times, a lesion will also form on the cat's tongue; usually near the back of the mouth. The exact cause of an indolent ulcer is not known although a genetic predisposition and a hypersensitivity reaction, such as flea-bite dermatitis, are highly suspected. This is the most common lesion of the eosinophilic diseases"

http://www.peteducation.com/article.cfm?c=1+2141&aid=3619