I am thinking of nailing down Trafficmaster brand engineered wood flooring. the overall thickness is 5/16. The instructions say to glue it down but I have a friend that said he nailed his and it's ok. Can this be done and what would be the best way?

We made this mistake, if it says to glue down do just that. If your subfloor is not flat which many are not, you will be pulling it up soon.

You also risk dimpling of the paper-thin top veneer that is typical of 5/16" engineered floors. Not having seen your friends floor myself, I know that people often overlook flaws in their own work; perhaps you would not think his floor is "okay" if you saw it yourself.