I so not know the specifics of how this was done, but using gauze could be used to displace lips and part of the cheeks (slightly from the irradiated volume).
This may be IMRT specific.

... of course, it is always possible that the person would not have gotten sores anyway regardless of gauze or not. FYI, I used cotton rolls and did not get sores where I placed them, although I got them in other places. Realistically this might also be depend on the precise location of the radiated volume.

M