I don't want to draw for my friend
Why don't you want to draw for your friend?
I've decided not to draw for anyone other than myself.
If that's what you want, so be it!  Nothing says you need to do any favors for your friends that you're uncomfortable doing.
Oh really? What if I change my mind and ask for you to draw for me and I'll want you to draw a lot.
I think it is within my right to turn you down as well. Just be honest with your friend about why you can't draw for him right now.