So the player of the wizard read the rules...
Example:
A spell with 10-shifts of power is being cast.
Her conviction is 5, she gets +1 bonus on power. So she crosses out the 4shift stress box
She rolls a +1 on a discipine of 5, get +2 bonus from focus items. Thats just 8, so she takes the 2 shifts as backlash, crossing the 2shift stress box (but not on the mental stress track.... she is a powergamer after all!)
The way I read the rules, she would have to absorb 6 points of stress in one go.
Who is right?
First, if her base Conviction for that element is 6, then pulling up 10 power would go past her stress track (the boxes being, in this case, 6,7,8,9).
But that said, yes, it's two separate stress hits, not one added up.
Wizards and spellcasters are always a question of resources -- yes, they can throw out huge attacks, but they have, at most, four shots in them before they start doing serious damage to themselves, whereas Johnny McSwordsalot can keep swinging forever.
There's lots of ways to balance casters in a party with non casters, but after 5 years? It might be time to finish the campaign and start fresh, especially if your other players effectively have no more advancement options. Better to start anew than to stagnate.