Looking for parts for an older 26 inch mongoose

This depends on what forks are fitted. Adjustment is usually done on the top of each leg. Look at the very top of each leg. if it is a flat plastic cap it is not adjustable.
If there is a twist top or a large nut shaped cap the forks will have some adjustment. Some forks can only be adjusted by changing the spring or Elastomer insert to a different rating.
These parts are usually quite cheap and can often be bought as a fork service kit.

Follow the link http://www.mongoose.com/usa/parts/tires?limit=all

Get the kind that has a lever shift instead of the grip shifter. The plastic part that holds the spring breaks easily. The lever shifter has been going strong on my bike for 6 years without a problem

It's not clear what she did to modify the height. Most strategies are here:

http://sheldonbrown.com/handsup.html

If you surf the web for "bicycle handlebar stem adjustments" you will eventually find something that looks like what it IS right now. I suspect you may want to UNDO the modification. You could engage a bicycle shop and save all the research, parts procurement and labor. OR go surfing.

Search the web for 'how to' do just about anything on a bicycle.

"adjust front brake on bicycle"

http://www.parktool.com/blog/repair-help/categories/brake-service-and-repair

Clues here...

http://www.parktool.com/blog/repair-help/categories/pedal-service

http://www.parktool.com/blog/repair-help/categories/crank-service

It's prabably easier to buy a whole new hub. And for ablout another $20 you get a whole new tire. You should do that. Have a bike shop do it because it can be tricky.

http://www.parktool.com/blog/repair-help/chain-installation-derailleur-bikes

Or it could be the dreaded mismatch of new chain with worn drivetrain parts...

http://sheldonbrown.com/chains.html

http://www.amazon.com/gp/product/images/B000B8IDS2/ref=dp_image_text_0?ie=UTF8&n=3375251&s=sporting-goods If it has a Shimano Altus rear derailleur, you can look here:http://bicycletutor.com/adjust-rear-derailleur/comment-page-3/ or here:http://www.parktool.com/repair/readhowto.asp?id=76

The bolt hole diameter and number of teeth on your chainring is all you need to know. These parts are universal to match the aftermarket standards foe cranks. 104mm (x4 holes) is the most common in newer bikes. You can look on eBay or any online bike store.

Regards,
matt