Ask HN: late co-founder underperforming

We have this terrible situation where our late co-founder is underperforming badly. I mean way below any expectations. But of course in our contract we don't have anything about expectations or performance and we are in Europe(Spain). Things are not going well with him, this was way harder than expected for him. And he is willing to leave the company, but he wants his shares to vest before the cliff period. Otherwise he is happy to stay in the company.

And it just feels bad to give someone 5% of the company because he was with us for 8 months and sold to 0 customers.

Is the only way to bite the bullet and give him 5% or is there anything else we can do?