Hello. I am wondering how to handle this installation. I am installing 1/2" engineered wood. The homeowner would like it nailed down. The existing subfloor is 1 1/2" fir, but has 1/2" particle board glued over it. What to do?

I would point out the extra labor involved in preparation for naildown.

Tear up the particle board. In which case is going to be extra labor intensive being that it's also glued down. THEN, once it's up, you run into the condition of the subfloor. Most likely it's going to be torn up , from tearing out the particle board. Who knows what's under the particle board, the fir subfloor may be out of tolerance for flatness, who knows why it was sheeted out. It's either going to take more labor to prepare the Fir. Or odds are good you will have to sheet over the fir with 1/2" ACX plywood.

If that's what they want, lot's of elbow greese, and go for it.

That's a situation begging to be a floated installation