My metalworker buddy took on a job to make and install a 7' x 12' sliding gate. The problem is he agreed to sheath both sides in wood. He knows nothing about working with wood, but he's also intent on doing the woodwork part himself.

He has decided on using pine boards, and he wants to glue the boards onto doug fir battens that will run vertically and will be captured within the metal frame of the gate. The glue will also need to work to adhere the pine to the metal frame.

What kind of glue should he use?

This is in Los Angeles, so the gate will be baked by the sun, but it will get some rain too.
I advised him against using pine, but he's set on it.

All advice welcome. Thanks.