Did the doctor give him any tablets to take? There are a few options for oral medications that he might ask about. Other options that are different then Lamisil. I'd highly recommend he call the doctor and let the doctor know about the blood sugar issues so they can work on another treatment path that doesn't mess with that balance. Other wise, make sure he's staying hydrated, keeping an eye on the carb intake, and keep up with the treatment.
If he thinks he may be getting it from where he is working, he may need to have a talk with someone in charge over the facility- this is something that puts people at risk and they should be aware of it.