Re: lighter load for the wife?
Forget re-barreling a 770. The barrel is pressed in, not threaded. It is as close to a disposable rifle as anyone makes. It is serviceable for what it is, but there is not much that can be done without going to a lot of expense, which defeats the purpose of the budget rifle in the first place.
If you want to change something, you are better off getting another rifle.
If you don't wish to get another rifle, I think judicious handloading would be the best way to solve your problem. I would start with some bullets in the 90gr to 110gr weight range. Check out the Hodgdon website for load data. They show both starting and max loads for multiple powders and multiple bullets. You should have no trouble finding something that works for your wife in that data.