Can I refuse to buy back my friend's share in our business?
Tell me about your reasoning. Why do you want to refuse to buy back your friend's shares?
He betrayed me when I was in dire need of money.
An eye for an eye makes the world blind. That won't get you anything except perhaps a new set of enemies.
I don't care about anything except my needs. He can suffer for all I care.
If you made a commitment to a friend, you should do your best to help them. If they betrayed you should talk to them to see if the regret it and you guys can make up.