No idea. The issue is actually complicated. The nitrocellulose and, if it is a double-base powder, the nitroglycerin will eventually break down, but there is both stabilizer (diphenylamine, usually) and deterrent (chemistry varies with the powder maker) as several percent or more of the weight. That is why smokeless powders have energy densities that can range from about 3600 J/g to over 5,000 J/g. It's the slower, single-base powders with a lot of deterrent chemistry in the first instance and the fast double-base powders with little by way of deterrents in the second instance, though graphite or other anti-static surface treatments also contribute some portion of the mass of the powder.
The nitrocellulose and nitroglycerin will ultimately break down spontaneously into carbon, oxygen, hydrogen and nitrogen in some combination (as CO, CO₂, water, NO₂ and N0₃, NH₃OH or NH₄ gas), but usually not until the stabilizer is consumed by the first acid radicals to get loose and unless bacteria that can break down and consume the deterrent and stabilizer is present. If it's not, the powder may remain relatively intact for decades.
If what your friend is making is a "hot" compost, the powder's contribution is likely to be poor. If it could harm anything or not I can't say for certain. It doesn't seem to bother lawns, as I've disposed of powder that way, though not before it was already breaking down, meaning the diphenylamine was already gone. Anyway, I think, unless your chickens are free to peck your lawn, that's where I would put it unless I could get the powder company to say for sure that it's harmless in compost that will grow vegetables to be consumed by people.