I think he's going to waste money on bed liner material.
I'm assuming he plans on doing each piece of carpet individually? If so, he'll lose the flexibility of the carpet which is going to make it a b**ch to remove and re-install - especially the pieces that go under the seats from the back or the one that gets tucked in under the console.
I also think he'll develop a lot more rust. Once water gets under them it won't have a chance to weep through the carpeting material and dry out. He won't even realize water is there because his carpet won't feel wet.
If he wants the benefits of both carpet and bed liner, he should bed line it properly and just put the carpet back over it.
Just my $0.02