you really shouldn't use green (wet) stickers, but we all do from time to time. It causes sticker shadow,a stain, and can promote rot where the sticker contact is located.
You are usually better off with stickers of the same species, but commercial stickers are pine and various other woods. Pine makes a good sticker. Many guys will get kiln dried wood from the dumpster of a building site and cut it into stickers. Plywood makes a pretty good sticker.
I would think that the your pine is usable, but warn him about the "shadow". He might avoid it by restickering a time or two to allow the stickers to dry some too.
