Welcome to Nico!
1. Spoiler - If it were me, I would check ebay and some other aftermarkets to see if I could find somebody selling the same spoiler, then ask them who makes it. Then contact them.
2. Not sure about the weather stripping. You might want to check out the service manuals to see how to remove it.
http://www.nicoclub.com/FSM/G35/ Next, check with Infiniti on a replacement piece.
Did you get him a Coupe or Sedan? What color? Auto or manual?