Guild Etiquette

After doing some AV'ing yesterday I decided to quit being a lowly lvl 60 and get on with the journey to endgame content and fabulous prices. So I flew back to Honor Hold and had the nice gem vendor fix me up after a few hours of Hordeslappin'. Fired Thottbot up, so I knew where to go and get my quests done (I'm a complete moron when it comes to reading the quest descriptions), and summoned my trusty Mistsaber.

Just when I hit 60 I was a bit disheartened by the huge jump in xp requirements to further levelling (from sub 200k to almost 500k) and had lost my motivation for a bit. After an hour I recalled how easy Hellfire Peninsulla quests are and was gaining experience like a schoolboy in a pornshop. Everything was going like cake and suddenly I got a whisper from a guildmember asking me to tank Ramparts for him. The guild is a fusion of several smaller danish guilds on the server and I wasn't sure who this guy was, but he was a fellow guildmember and I could do with some more Outland tanking practice. I had tanked BRD for him once before, but he had suddenly skipped the party mid-instance back then, which I found a bit strange, but gave him the benefit of the doubt. The man probobly had his reasons.
So HS to Honor Hold to repair and shop water and talbuk meat, and then on my way to the summoning stone.

We get the party summoned and I look forward to working with the guy. He's a resto druid so we'd be a fairly good couple if we'd level together and do instances along the way. We go in and do the first few pulls with no real trouble. The Pug seem to work together ok: Everyone follows marking, let me grab aggro and a few even sport Omen - not my guildmember though, but no biggie. Suddenly he announces he has got to go and without further ado he leaves the party. The others are a bit miffed and so am I but he probobly had something urgent come up. After five minutes of waiting for a replacement I notice he's still online and whispers to him to ask what the trouble was. He tells me he'd gotten an offer to heal an Underbug run - and who would say no to that? "Ehhh, most people, I hope" I respond. I know I would, and tell him I think it's plainly rude to the rest of the party. He whisper me a respons along the lines of "yeah whatever", and I get a bit pissed. I remind him of the BRD run and tell him I find it rather disconcerning to do so in a Pug, but very much so when you'd called in a guildmember. His response? "Then just don't trust me in the future, mate...". Ehh, wtf?

So, we get together a new group and blast through the instance twice with good loot to boot, but I can't really let go of the feeling he left me with. I whisper a mate of mine who'd been in the old guild considerably longer than me and describes the situation for him. He tells me to take it up with an officer, but, sheesh, it sure feels like running to a grown up and ratting when we were kids. Still, if we we're kids he'd been on the recieving end of good dose of whopass by now, but alas the internet has no /punchfist to face function.

So what do I do? Rat on the rat, or just leave him to rest on my ignore list? It feels a bit low, but a combination of a lust for revenge and urge to protect my fellow guildmembers from his apparant apathy drags me to it (ok, mostly the lust for revenge bit).
What's the normal thing to do here?