Let me begin by wishing your Father-in-Law a good outcome.
As BB Legends Don and Lokki have observed in considerable detail, the road to safe streetability for your Spider will almost surely be long and paved with $$$ and I fear the cost of professional intervention will likely prove a major impediment to even minimal roadworthiness. Like many others on AlfaBB, I would never have been able to afford Bella without my own wrenching, although perhaps surprisingly the cost of essential parts, while cumulatively significant, was manageable over my many years of ownership.
You mentioned the “...car drove fine at the time...”, which is certainly encouraging, although signs of neglect and deferred maintenance obviously abound in your excellent photos. Overtightened valve cover gaskets (oil in plug wells), a failed front crankshaft seal (that Exxon Valdez level oil mess at the front of the engine) and other apparently defective seals and fluid leaks indicate your FiL, like those car enthusiasts who mainly enjoy driving, was not overly troubled by cosmetics. As long as you don’t look under the hood often, park over cardboard and the car soldiers on, many of the issues in your photographs can be endured.
My approach would be to follow Lokki’s advice and begin by determining the condition of the engine, specifically if it is frozen.
Without investing in a new battery, it is possible to turn a non-seized engine (plugs out) by using a breaker bar on the large nut which secures the vibration damper to the front of the crankshaft. Access from above is difficult and putting the car on jack stands is advised and should be something you can do in the garage. Of course, if your FiL added an oil pan guard, that will need to come off too. Don’t lift the car too high, or you will bump into that “parking assist” hanging from the ceiling.
“First, do no harm.”
I’ve suggested checking the engine as a first step, since if it is found to be seized, all the downsides Lokki mentioned (rusted liners to be replaced, etc.) would necessitate pulling the engine. And as with nearly all vintage machinery, additional discoveries would certainly present, adding quickly to your expense.
In conclusion, I believe a frozen engine would be my signal to proclaim “Check please!”.
Maybe the engine will turn easily and a flush of the fuel system, from tank to injectors, a new battery (or a jump?) may bring her to life. (Since you are only performing a brief “reality check” by idling for a few minutes, I wouldn’t bother doing coolant and oil R&R...just make sure there’s enough of both in there. And keep RPM under 1500!).
Actually, a running engine might ultimately be a bad thing since, after determining the engines idles without overheating, and the clutch and brakes have a degree of function, you might be tempted to make a short jaunt about the neighborhood.
After that, I fear you will be hooked on Alfas and there will be no turning back!
Keep us posted and welcome to the Forum.
OK, signing off.