Shredded aspen tends to be pretty soft, relatively speaking. Even a piece that looks "toothpicky" will likely bend and fold easily. The snake's digestive system is created in such a way that it can break down and disolve hair, fur, feathers, bone, cartilege, etc. A little piece of aspen doesn't stand a chance in there.

That being said...if I saw a snake in the process of swallowing a meal and a large piece of wood was caught along with the prey...I'd probably reach in and try to pull it out while the snake was still swallowing....but only with a very gentle tug to see if it would slide out. I wouldn't force anything. Once they've bitten and constricted the prey...they are usually in a sort of reflex-driven trance until the prey is swallowed and will not be bothered by a gentle intervention at that time.

Hope that helps some. If you think the piece is still stuck in his mouth, then I'd recommend finding either a vet or an experienced snake keeper in your area to help fish it out.