The problem with mold is that it may simply go dormant rather than die in either extreme of hot and cold. The same can happen drying it out. It can't grow or reproduce but go out in the rain and forget about it when you come home and you could have a new outbreak. Better to find something that kills it outright. I've had two stocks that had mold, the first was unsalvageable and was on the Swedish M38 I brought back from the dead a few years ago. The second was on the No 4 that I got with the Swede. I used a combination of citristrip and wood bleach to kill it and remove the stains. This was a drastic cleaning and removes all of the original finish and may not be necessary on your friends rifle. No 4 looks great today after numerous coats of BLO. Swedish stock I didn't really mess with much. Snapped in half about 5 inches from the end due to combination of the mold and dry rot. I cut the worst off and kept the shortened stock for I have no idea what reason other than reluctance to throw anything away. I could never get all the stains out of that one.