You move her/him into an individual contributor role. Or just move her/him out.
This is one of the most frustrating outcomes, when a very talented co-founder becomes “unrealiable”.
What that really usually means is they just aren’t as committed as you are. And oftentimes, that’s completely logical. Startups are so hard, and it’s so hard to see success in the early days. It’s logical one co-founder may be less committed, and become “unreliable”. Especially if you are working for $0 or a very low salary.
It’s almost un-fixable, however. You can try to transition them out. Or you should at least not have them manage anything. That lack of reliability will translate into a toxic culture of folks who are less than 100% committed.