You seem unsure about these issues and you're dubious of your Son's abilities
... Well that is Okay... as long as you accurately portray whether or not you feel you have the Skill Set necessary to perform the repairs that may be required as per whatever follows. There are quite a few GMTN Members with an Expert Level of HVAC Skill ready and willing to help and advise you along the way:
In a properly sealed A/C System, there is always System Pressure inside that gradually balances out between the High and Low sides as the refrigerant re-distributes itself. The Manifold Gauges will shows these two coming into rough balance AFTER the Engine has been turned off for a while.
Of course this dynamic changes whenever the Engine Starts and the Magnetic Clutch engages and the Compressor begins to work. The Manifold Gauges will show that the Pressure on the (Blue) Low Side Vapor Return Line side Decreases while the (Red) High Side Liquid Line side Pressure Increases as the Compressor keeps on running. This is Normal A/C functionality on a working system.
However, with the Engine turned OFF, if the entire A/C system is either empty of Refrigerant or fairly close to it, then the Manifold Gauges will STILL show the actual system condition as having Very Low Pressures on BOTH sides. Variable Internal Line Pressures and Temperatures (inside and out) are always intimately involved in all uses of all types of HVAC Refrigerants, but the Compressor in your Saab will NEVER engage if the R-134A has been reduced below 40 PSI or has become completely lost from within the closed A/C system.
So first, you will also need to determine if you have any Refrigerant Leaks that have lead up to this problem,
remembering your Wife's earlier advisement of having "Low Cooling" as the most significant indicator. The Best Tool for this part of the investigation is to get an inexpensive Refrigerant Leak Detector available on Amazon:
Amazon.com: Elitech WJL-6000 Freon Leak Detector Halogen Leak Detector Refrigerant Gas HVAC R22 R410A R134A R1234YF CFCs HCFCs HFCs Detector High Accuracy: Automotive
www.amazon.com
Assuming that this problem turns out to have a simple in solution
(...like a Leaking Seal around a Bad A/C Low Pressure Switch for example) and you manage to isolate and repair any such Leaks that you find...
AND the Compressor Engages, then the most direct way to determine what the actual R-134A Charge left inside the system is to literally: Recover the remaining R-134A Charge and then Weigh-In a Brand New Charge of R-134A (in Pounds) according to the amount specified in the Saab Service Manual.
Barring that approach, you would require some level of experience and sophistication with using and interpreting the Auto A/C Manifold Gauges and Tools along with knowing what the Ambient Temperature is once the system is up and running to then figure out if the A/C Charge inside is either Too Low, Correct or even Over-Charged. If you have this Skill Set, you can download and use this Pressure vs. Temperature Chart to help you: