Having been a startup founder on the other end of the equation you’re describing, my advice is to be generous. Equity should be earned, not given and your partner is still at the beginning of what is likely going to be at least a five year journey. The fact that you didn’t have a vesting schedule just speaks to inexperience.
Also, don’t discount the demotivating effect all the deadweight your equity can have on the team. The success of this startup is far from secure, and if there’s too much dead weight on the cap table, there’s a very real chance your ex partner decides it’s not worth it to keep fighting on this hill and walks away, making everyone’s equity, including yours, worthless.
My advice is to give away what they’re asking in order to maximize your chances of seeing a payoff one fine future day. And because I, personally, believe it’s probably the most fair thing to do.
OP says he’s only worked there for 1.5 years, so with four year vesting he should only get 37.5% theoretically of the total equity he’s been allocated.
Also, don’t discount the demotivating effect all the deadweight your equity can have on the team. The success of this startup is far from secure, and if there’s too much dead weight on the cap table, there’s a very real chance your ex partner decides it’s not worth it to keep fighting on this hill and walks away, making everyone’s equity, including yours, worthless.
My advice is to give away what they’re asking in order to maximize your chances of seeing a payoff one fine future day. And because I, personally, believe it’s probably the most fair thing to do.