First I have heard this, but heard a few get these sores from drinking pineapple.
I see such sores as a cleansing effect and I suggest as BOOK I suggest, drink your juice diluted with water and on an empty stomach and do their 14 day fast with the epsoms salts for the first 3 days and get the toxins washed out and then fasting opens the channels that let these acids come down and out to the elimination organs and not out the skin/mouth, etc...
Is it possible a person is allergic to fruits? Yes, but not natural. I was allergic to coconut for 45 years and after doing a 40 day orange juice fast coconut hasn't bothered me since; so I know coconut was not the problem, I was.
The pre-cleanse and liver flushing etc..all helps open the channeles of elimination up as does following the food instructions of BOOK I and herbal info of Dr. Christopher.