I strongly suggest that he check with an experienced Taxidermist. Hides for a mount are tanned differently for that purpose than those tanned for clothing or for other purposes, even a rug is handled differently. Mink oil, while a preservative will soften leather and probably cause it to stretch and thus make the mount fit incorrectly. Just what is wrong with it anyway? How long has he had it? What are the humidity conditions in his home? Has it been exposed to any chemicals fumes or other caustic substances? Those are only a few considerations that have to be checked out. The Taxidermist that did the original work might well be the best source of an answer if he is still in the area or contact a Taxidermy supply outfit like McKenzie.