The main problem your bf may have is that bikes from box stores generally come in one size - 'MEDIUM' - and he should be riding what most manufacturers call an 'extra large.' Riding the box store bike will be less comfortable, less fun, and more frustrating than riding any bike that fits.

If you keep your 29er and he gets a used 10-speed with a 60 or 62cm frame, he will feel like a champion when you go riding because he will move faster with the same effort.

I have heard some people say heavy slow bikes are better for exercise because you actually spend more energy keeping them moving, but in reality all that happens is people tend to ride less because it is less fun.
THe same goes for you - if you are riding mostly on paved or at worst gravel or crushed stone surfaces (road and bike paths) then you should replace your tires with road tires - narrower tires with minimal tread designed for higher pressures will give better traction (increased safety) and better efficiency (longer distances and higher speed for more fun and more exercise). 700 X 35 or 700 X 38 tires will be a major improvement over whatever came stock on your bike.