That is a touchy situation but you need to make hard decisions when involved in a startup. I am assuming the new Technical co-founder has more experience than the original team member. If that assumption is true, I would sit him/her down and say, as you know 25% of a small company is not much, but 5% of a huge company is significant. If we can attract this new CTO, I believe we can take this to the next level. I need you of all people to be onboard with this and be my eyes and ears when it comes to the engineering team.