Without knowing the work your son in law has done,I can suggest some things to check...he may have already eliminated some of these. First let me tell you a couple of things...1) I am NOT a certified mechanic...I am an autocad technician 2)I am a Mustang Gal and know Fords far better than anything and 3)I am assuming that you have an automatic transmission.
Here's what I would start checking
*O2 sensors (longshot and should be throwing a code if one is bad)
*throttle cables to see if any are stuck, especially if he had the trottle body off
*vacuum cables...if any of them are leaking/not sealed properly you will have idle issues
*if there is a check engine light, I would suggest pulling the codes. That should tell you what it is right there.
*some part of the air intake stuck open
My boyfriend has a co-worker with a saturn (also made by GM) that had a high idle problem...this is the first thing I would try because it's a cheap easy fix if that's it. She cleaned part of her throttle body and the problem went away for a short time, so she took the whole thing off and cleaned it....problem solved!
Good luck with your problem.
L.