The diatoms are dead material. You buy them from your LFS
They are the shells from microscopic critters and are fine like baby powder.
One word of caution, don't let it get into your tank. A very small amount won't hurt but 1/2 tsp. is too much. It's sharp and will cut your fish and snails.
What brand is the unit? Depending on the brand, the motor is the only thing that can wear out.
How old is it? How often was it used? The motor will last a long time if it's not over heated by running too long at a time.
One brand has a filter cartridge that needs replacing after a couple of years of steady use, the other has a bag ( I believe) that the powder goes in. That bag might get holes in it from the diatoms, they are very sharp.
The only way to test it is to run it. Have them set it up on their tank with 1/2 cup of powder and watch it run.
You shouldn't see any "dust" or particals coming out after the initial start up.
You can only run it 4 hrs at a time and you don't need to do it more than a couple of times a month, if that.
It's a very fine filter and is not used for primary filtering. Basically you are filtering "clean water" to make it extra clean.
It does not effect CO2