I'm going to try to stop my friend from putting more money into a trading business that seems like a scam
Why get involved in your friend's trading business?
Because I'm worried he'll lose all of his money.
If it seems too good to be true, it probably is. It is good that you are trying to prevent your friend from possibly getting scammed.
I didn't listen and kept warning him about it. He's not happy about it but he should learn his lesson, I can't let a friend of mine lose their money.
It's good to look out for friends to keep them from getting scammed. Just be aware that you're not overstepping any boundaries. If they feel like you are they might be less willing to listen to you.