Can a business partner claim back investment made if no contract?

I think you can fight about it, but if your business becomes successful you will wish you didn't. In similar situations before, I have just bitten the bullet, paid him off, and gotten on with life. I'm glad I did it. (The younger crowd who have never been bloodied in legal fights will tell you to fight it. I think that inviting legal fights into your life is the stupidest possible thing you can do).