To be fair this is why you don't trust your friends blindly. Why 30%? If you want give a 10% discount on the order amount and be straightforward with him citing you'll be in legal troubles. I would simply decline his requests from now on to use your card and account.
I have friends who ask to use my card, I give it to them and don't take any cuts. Again they are friends whom I know for a very long time and they always clear their dues well ahead of time. They help me to increase my overall spend and in return I don't take advantage of it. But that's just me. If I wanted a favour from them, they'd gladly agree without a second thought. Keep friends in life who are not cheap skates intentionally. Friends who are not doing well financially is a totally different case. Not the ones you have described in your post.