Its all about not pushing them to take on your advice, I always give them some tips to get rid off the biggest flaws in their builds and tell them if they ever want advice, just shoot a tell and il take my time to help them out. Best way imo to let people realise that their toon could be much more then their playing with atm.
On topic, as for the first post regarding research. Im sorry to say but the forums aren't really that reliable either when it comes to building a toon. I have seen a couple of builds on here that where flawed with their feat selections. Therefor I only look on the forum for inspiration and then alter the build to my own playing style.
As for strange builds, I find the WF ranger / pally / rogue splash of my guildie Crossy to be somewhat weird. Even more cuz he always uses 2 handers but then again once you play with him you notice that it works. . in his hands that is. I dont write off weird builds straightaway, the build and gear isn't even half the thing that makes the toon, gaming skills can make up for allot of the gimpness that we might perceive about the build =).
Thats why I never denied a toon with a exotic build, I do make a mental note of the players name if I see him fail utterly in multiple quests but thats about it. Also, I dont grade a toon its effectiveness based upon the amount of kills he makes. My wizard can come out as a top killer in allot of groups by simply nuking and instantkilling everything but allot of times I prefer to toy around a bit and use as much CC as possible. Differen't approach but both just as effective [depending upon the quest some situations demand one out of 2 approaches but their mostly both valid approaches.]
You just seem to have had the bad luck to end up with a player that indeed had a flawed build and lacking the skills to make up for it. By my guessing he was to stubborn to take on advice, if that, coupled with the fact he died allot and didn't contribute much to the group caused you to rage thats perfectly understandeble. Heres my tip for you when it happens, go smoke a cigarette and continue playing once your calmed down again a bit. That always works for me =p.