Helping other people with character design should always be done very carefully, too easy to take their sense of investment away.

That said, just steer the non optimizers to the good core classes wizards, clerics, druids and rogues are hard to screw up (well the rogue needs dex and his skillpoints in the right place, but that's pretty straightforward). Unless the optimizers are using cheese (incantatrix, artificer, celerity, uber charger that sort of stuff) there really shouldn't be a problem then.