Let it go. Let him hire the engineer he wants to.
And then when it doesn’t work out in 3–6 months, hopefully, he’ll see you were right.
The CEO is the CEO. Everyone smart around the table, the co-founders, the CTO, the investors will “see” mistakes she mistakes. She’ll make a lot.
But the CEO has to make a call. The buck stop there. So sometimes, as CEO, you’ll just think you have to do something everyone else disagrees with.
Support her. Let her make the “wrong” hire. You’ve had your say. Good that you are passionate here. But let it go at this point.
Similarly, as CEO, you have to let your VPs make a bad hire or two, as well.
But if it’s 5, 10 bad mistakes in a row. Then, it’s time to move on … and find your next gig.