Car shakes when idles

My sister has a domestic car that is about 15 years old. Her car has started to shake when it is in idle. It doesnt shake when she is moving. We dont live near one another so I havnt experienced the car shaking. She has had a tune up and the car still shakes. One shop she went to hooked her car up for a diagnostic and told her nothing is wrong and charged her $300.00 for the diagnostic. She feels ridiculous that she paid for the diagnostic. Another mechanic she went to said he would tighten,fix, or replace the mounts and said that MIGHT fix the problem and that the part would cost $190.00. Could this shaking be a mount problem? What is a mount problem? I dont think any of these mechanics have even been in the car to feel the shaking. The car doesnt die when it is at a stop it just shakes. What is wrong with this car? Im going to recommend that she go to a mechanic in her area that has positive reviews on this site but in the meantime any thoughts on this shaking car?

The second mechanic was going to be my suggestion. It is a motor mount.  If the motor mount(s) are bad then they could cause that problem as could the need for a tune up.  

I suggest you start by posting more information.  How long has this been going on?  Did it start suddenly?  Is it getting worse? What make model and year car do you have?  Is it automatic or manual.  If auto, does it do the same thing in neutral as in gear?  Any history of other problems, even if they don't seem to be related? 

The second mechanic should be commended for saying it MIGHT fix it.  Even if the mounts are bad, it is still possible it is something else, but bad mounts should always be fixed.

This is a 2-person job.
Ask someone (or you) to look at the engine vibration while another person (or you) pump the gas pedal lightly at first, then mid-level, & fully. Compare the engine movement. If it’s motor mount, you will notice the engine will move quite farther than it should be.