"An issue I have with this is that one reason my co-founder wants to split is that they don't really want the pressure of running a startup"
Whereas it sounds like you have a solid plan to grow the business and generate revenue. As @jedberg says, this makes your interests aligned with the (neutral) investor, and the other founder at odds with the investor.
a) You and your cofounder can't work together
b) One must leave
The investor and one founder can push out the other founder. Who do you think the investor thinks should leave if you reread your quote above?
Whereas it sounds like you have a solid plan to grow the business and generate revenue. As @jedberg says, this makes your interests aligned with the (neutral) investor, and the other founder at odds with the investor.
a) You and your cofounder can't work together
b) One must leave
The investor and one founder can push out the other founder. Who do you think the investor thinks should leave if you reread your quote above?