Do you have any other clients with Pine Planks that are finished as you are looking to have done to Bob's planks? If so, ask 2 or 3 of these clients if they'd allow you '1 last shot' at trying to disuade Bob at covering over his planks.
W/O knowing much about the bigger picture, I'm sure there's a cost issue too. In short, it's 'cheaper' to cover them with Pergo than it is to finish/refinish the existing floor. Although I'd be against covering the floor, you're best to present your client with his 'options' and then be done with it. Perhaps you could look a little harder at the existing floor. If it 'slants' or has 'dips' in it, Pergo isn't going to be a good option because it's installed w/glue. And with an uneven floor, they will most likely 'separate' over time. Also, you may be able to tell him that if he wants to go w/Pergo, he's probably gonna have to install an underlayment to 'level' the floor anyways. This expense, added in with the cost of the floor, added in with the cost of labor will PROBABLY cost the same price to finish/refinish the existing floor.
Now, at the same time, if cost ISN'T an issue but 'LOOKS' is the issue, then probably no matter how hard you try, you won't be able to convince him to go your way. As I said earlier, at some point, you have to say to yourself, "I've tried and I have to move on." AND, if his 'taste' is in line with having pine planks, then like you, I too am baffled as to why he wants to cover them. What I mean is, a restored Farm House goes nicely with Pine Planks.
I do with you well. My best to ya and hope this helps.
Jay J -Moderator
PS: The innate beauty of pine planks, flaws and all, is VERY appealing to many people who are into that type of flooring. I call this 'character' ...