Let her play whatever character she wants.
Be firm. Don't let her walk all over you. See if she wants to be a number cruncher and work it out herself or if she just wants to roleplay for some good old fashioned escape-ism. Either way, make sure she advances by the rules.
Make up three other PCs. One of each of the other major archtypes. If she's a cleric, make a wizard, a rogue, and a fighter. Keep them simple and avoid TO like the plague. She's the star, the NPC party members are there to allow the party to adapt. Base class only.
Encourage her to figure out ways to twist the rules. Point out things like being a cloistered cleric and that she can give up domains for domain feats. Encourage her to become CoDzilla. But start at 1st level. Make sure she earns every XP and GP.
Kill her if the dice say so.
Create a NPC who's a Bard. He never adventures, He's the party Fixer. He arranges the adventures and promotes the party so that they have a good rep. Think Hollywood Agent. He gets a cut of the treasure. 1/2 share. So you divide everything 4.5 ways. He stocks it away and in the advent the party gets slaughtered, he pops the 4k GP to have them all reincarnated. How'd he pull it off? He's a sneaky bard and has other adventures working for him. THEY are the ones who recovered the bodies. He is always 1/2 the level of the PCs. He also pays a lot of the bills, taxes, adventuring licences and fees. He covers all the party's other expensives so the PCs don't have to worry about that crap.
Knock her down a level and make her roll a new race. She might get grumpy, but if she actually realizes you'd kill her, her victories will mean more.
The dice are law.
Don't be afraid to kill her, but also don't be afraid to lose. That's the major problem with DMs, they think it's you verses the PCs. It's not. You are there to be the impartial ref as they travel through a world of your creation. Your goal is nothing more then to be entertaining.
Don't Railroad.
If you railroad, force the players to follow your plot, they will hate you. Allow her to ignore your plot and run off in a weird direction. I have one player who has avoided her Plotline for months now. I'm serious. We spend more time shopping then we do adventuring. It's okay. She's having fun and when the other players show up they've grown kinda sick and tired of World Spanning Horrors. (Avg Level is currently 27th) They just spent the last adventure trying to figure out how to negotiate a treaty between a lake full of intelligent beavers and a Great Wyrm Blue Dragon who's trying to raise a crop of Giant Mushrooms and wants Said Water for his crops. Not a sword was swung, not a spell cast. They had fun just dealing with the insanity of it all.
I won't mention the actual plot, they read the board sometimes.
Adapt. Be prepared to throw everything out the window. Allow the players to completely end run you and win if they think up something good. You can always make more badguys, you can't always make happy players.
Players don't want funky cool powers, it's up to you to explain that to them.
Players want to defeat the enemy. They want to save the day. They want to be heroes. Your purpose is to make them FEEL like heroes. Powers don't make a hero. What a hero DOES makes a hero. If there is a NPC kid so unpopular that people line up to spit on him as he walks past them and they players figure out how to help him out, that will mean more then all the Caster Levels in the world.
I had an entire adventure around trying to save a little child's kitten who wandered through a random gate into Undermountain. The reward? One Silver Piece. It was everything the kid had and she offered to hire the Adventures with it to save her kitten.
I've had villians the players hated so much they were frickin jumping up and down when they FINALLY destroyed him. It cost them so much to pull it off, but the joy of splattering him, and having to work so HARD to do it, is what made it worth it.
But if you don't seem impartial. If you fudge the dice rolls, even once, and they find out you 'saved' them. Your wife will never forgive you. She'll wonder if you 'Let' her win. It's about journey, not the outcome.