I don't know. The advertising copy makes it sound pretty water resistant. And your daughter can always keep it treated with Sno Seal. People have made leather holsters work in AK. Heck, a friend of mine even took a blued revolver on a whitewater trip during rainy weather. It kept him busy cleaning off rust, oiling, etc. but the gun survived the experience. However, I prefer minimal maintenance equipment because on an AK wilderness trip you always seem to have plenty of things to keep you busy.
Lawman sounds like a well-established company with a good reputation. Hopefully they would take care of any problems if the STU did not hold up to your daughter's needs. If you go with leather, that sounds like a reasonable option.
DancesWithKnives