Can you sell your friend your old 2GB modules?
Yes, absolutely, free market and all that...
Will he run into problems getting it serviced from Apple? Tough to say. I've heard people complain about running into problems like that. I think it is outrageous, since Apple designed the notebook to be user upgradable in respect to RAM, and also publishes upgrade instructions in the documentation that came with the computer.
Usually people recommend holding on to your old RAM in case you need to install it again prior to servicing. How much can you get for 2 GB of used RAM? $20? I'd probably just hold on to it as an insurance policy. Or sell it to your friend with the understanding you may "borrow" it if you need to send your machine into Apple.