I would say the best thing you could do is take what you can from this situation to strengthen your guild and its policies.
When someone "emo rage quits", meaning they don't talk about their reasons for leaving first, all of their characters should be instantly kicked. Additionally they should not be invited back for any reason (though there may be some extreme circumstance, but that's rare). People that you want to take back are ones that take the time to explain why they are leaving. You want rational, cool headed people around you. Especially the ones who are officers.
Establish your loot rules before loot drops. No matter what your doing 10, 25, new, old, it dosent matter. Lay out the rules before you start and Do Not Alter them mid run. It only causes drama and puts you in a bad spot.
Last night I almost had a situation along the same lines. I was running my usual off night ICC 10. We had 1 regular and 2 backups that were unavailable, so we brought in a PuG. Long story short a Staff dropped that would have been a big upgrade to a member thus furthering the guild in the 10 run and more importantly our 25 run. It went to the PuG that was there for 1 night. I started to have side conversations with the people involved to decide if I should ask the PuG to pass it to a member. In the end I kept to my original loot rules and the PuG kept it, without knowing the behind the scenes conversation.
In the future on the rare occasion where I have to PuG a slot I think I will say: "You are welcome to greed on anything you like, however this run is meant to gear our members so on some specific items they will have priority to need over you." Slightly unfair I know, but the purpose stands that's what were there for. The PuG still gets Rep,Frosts, and a decent shot at some loot since most of us are half 264 anyway.
There is something so appealing about backhanding someone across the face with a shield.