Does your MIL know what it is that she aspirated? and does she know when it happened?
There seems to be quite a difference in severity of this type. It can be very serious, or not so much. I think maybe because of what has entered into the lungs, and the person's level of lung function.
I've had aspiration pneumonia. The first time it was almost like walking pneumonia, I gradually got achy and sick feeling, eventually ran a high fever. I felt like I had something down in my chest and was short of breath. Antibiotics took care if it right away. The second time, I didn't run such a high fever, but it was harder to breathe and harder to get rid of. Both times it was only a tiny amount of fluids that had gotten in there, and it was also not something that had been in my stomach first, which probably helped too. I was not hospitalized either time, and did not have much phlegm from it.
They tell me down at Hopkins that they feel being proactive with anything involving the lungs is the number one way to keep a PAL as healthy as possible.
I hope your MIL does well once they have her on some meds. They will probably give her breathing exercises to do as well.