ya it'd be fair cuz that lv 120 took the risk of jeopardizing his guild rankings. Even if it was for fun, you can do that via team pvp.
you want an organized game, you need organized players in your guild. If someone's stupid enough to do something like that, warn them within your guild if you feel you actually care that much about gvg, and then kick them or w.e you feel necessary.
its guild vs. guild. The leader doesn't have to be present, just 2 groups willing to risk their rankings. Regardless of what's fair and what's not, you have to decide that and know you're a part of your guild, and your actions will reflect guild stats.