How to make my wife's bike lighter? Is it worth the cost? We inherited a Huffy Stone Mountain 26" mountain bike. We live on the 3rd floor of a walkup. It is quite difficult for her to carry the bike up and down the stairs.  It has a fork with a spring suspension. Other parts of the bike look normal. When I pick up the bike, it seems to be front heavy.
I'm not sure how much the bike is worth now...maybe $50 bucks maybe a little more? Is it worth trying to tweak it to make it lighter or should I just trade this one in?
Difficult to say without knowing more about the bike, but I'd trade it in for a better bike; Huffy has a rep for low-end, tank-like bikes. From your description, you'd need to replace the frame and front fork to see any significant weight savings. 

Something with an aluminum frame like a hybrid bike would probably be easier to carry up the stairs, even though aluminum will give you a harsher ride. 

Suspension forks are heavy, particularly on low-end bikes. Suspension usually makes sense for off-road riding. If your wife mostly rides on pavement, you can get a bike without suspension of any kind.