So that's a respectable outfit? (Not being offensive, I just don't know, but it sounds very legit) If it is, then as the others have mentioned you may need to do a methodical elimination of components that make up e-juice. First - as one person mentioned, does this happen with certain foods or drink too i.e/ if she eats a sour candy of any kind? Is it only on one cheek? It's weird but I always get what feels like raw discs on the roof of my mouth when I eat crusty bread for example)
Did you find anything new relating to the incident between postings? Otherwise, I'd suggest getting some VG and have her vape only that for a few days. Then I'd suggest veering away from citrus or cinnamon, and/or trying a different juice from another vendor you feel is good quality. It could be a flavoring, it could be the nicotine base used, it could be PG (I used to get a hard dry hacking cough, and felt my mouth and throat were "raw-ish". Or here's one, does she concentrate the vape to one side of her mouth specifically?
Does she drink enough water? - that's key because you've got to stay hydrated. All of these are only thoughts and ideas for you both to get to the bottom of this. Not having any medical flair, it's probably best she see a doc if it's bothering her as at least one medical condition was mentioned in a prior post that would affect only one side of the face, other than dental conditions. Good luck and let us know how you progress!