[iancmceachern]

If your co-founder "decided" without you, you aren't co-founders and he/she doesn't view or respect you as a true partner.

If they viewed you as a true co-founder and partner they would have phrased it as an ask. "Hey, I've been pondering pivoting us to this, what do you think, should we consider it?".

In the end it's so much more than the idea, it's about the team. If your co-founder doesn't view you in that way then that's not really a problem you can solve.