That reminded me of a plot important NPC who was a young boy in my game. I basically treated him like a familiar (see Arcane Power or DDi glossary) with a passive mode (in which he couldn't get hurt) and an active mode. I'm going to suggest you treat the wizard PC similar to how I did my NPC boy - as a familiar in passive mode.
Now, spy our wizard PC has firepower, and to represent that I would work out her DPR (average damage per round), then just apply that to the enemies each round the wizard fights as makes sense in your narrative. Don't worry about tracking her HP and, for the most part, forget about her defenses. Narrate her spell damage however is appropriate for the PC, arc lightning, fireballs, magic missile, whatever. If you want her to cast a signature spell that is control and not damage (like sleep), or to cast a ritual, then you can consult the sheet.
Treat threats to the wizard as a narrative motivator for other PCs. For example, say the party is exploring a catacomb and forget to guard their flank when they encounter some treasure, then the wizard gets ambushed...you could say something like: "The creature wounds the wizard and pins her against the wall, drawing a spectral blade. The wizard has no place to maneuver...unless you separate them quick, it's going to get ugly for the wizard!" Then follow thru on the threat if the PCs don't act, knocking the wizard out of commission (and the narrated damage she deals) until she is healed up.
I know it's fast & loose, but that's the idea. Less for you to think about as DM, and feels a lot like how a wizard (glass cannon) is treated in combat.