Co-founder wants to leave, do I pay him back the money he invested?
Since he now wants to leave, should I pay him back all the costs he put into the business? (we have no contract/agreement stating our equity split)
Since he now wants to leave, should I pay him back all the costs he put into the business? (we have no contract/agreement stating our equity split)
You probably DO have a contract stating your equity split - in UK law it is not necessary for a contract to be written in order that it is enforceable. If he were to seek redress in the courts, a likely outcome is that they would order the winding down of the company and he would be entitled to 40% of the proceeds (if any), less the outstanding capital he owes the company.
But you're asking what you should do, so I think the easiest option in this case is to come to an equitable agreement with your co-founder that you are both happy with, and save yourselves a lot of hassle. If necessary bring in a neutral third-party "arbitrator" (mutual friend or someone you both respect) to ensure this is done fairly.
DISCLAIMER - I'm not qualified to practice law.