Low oil should not cause a head gasket to blow unless there was engine overheating involved. However, an engine struggling to run because of low oil and trying to seize up can overheat if kept in operation.
That 1.5 quarts of oil remaining is not enough to protect the engine. The oil pressure light will go out with as little as 3-5 PSI of oil pressure but that small amount is not enough to protect the engine. It may keep it from locking up but it’s being damaged in the process.
This may be tough to sort out about who’s going to be responsbile. What would I recommend as a first step?
Get it out of the hands of the people who botched the oil change and have a vested interest in this.
Have an oil pressure check performed along with a dry and wet compression test. Those tests will determine (sort of…) the extent of any damage.
It makes no financial sense to invest money in a head gasket job if the engine lower end is damaged due to lack of oil.