I would hate if anyone did this to me, let me win fair and square, don't try to build up my confidence that way. I would rather earn it the right way, I'm not a 5 year old kid that you have to "let" win.
Plus I know enough about bikes that I would probably figure out that you "let" me win.
If you're going to make your wife any stronger and more confident, be honest with her right from the start. Make her EAT YOUR DUST!