What are the best things I can do to support a friend and his family during a deployment?

Capt Brandon Charters I would establish a solid rapport with close friend and family. Just communicate and a solid care package goes a long way too.

Offer to help with things he normally would have done; mow the lawn, scoop the walk, take the kids to school, and help out with the things his wife feels weighed down with. You won't take his place, but you can help cover down on the things that would otherwise overwhelm her.

The Marine Corps had a family readiness officer. Usually this position was held by one of the officer's wives. Their role was to build a support network amongst all of the families with loved ones overseas.
Sometimes they would have parties where they got together to create something to send to their loved ones. Or maybe the deployed loved ones created a video for the families to see. I missed two Christmases in a row overseas. We always sent a Merry Christmas video home.
Maybe you can find out if his detachment has something like this setup.