Sometimes you do not have the luxury of waiting to treat until you have all information. In general a short course of steroids is extremely safe. Additionally, steroids are correct to give in CNS helminth infections anyways, although usually it is dexamethasone, not prednisolone. More likely the mycophenolate was the immunosuppressant that let it get into the brain not the steroid.
More importantly, they did quite a bit of workup, including bronchoscopy with BAL, which indicated she likely had a form of eosinophilic pneumonia, probably Churg-Strauss syndrome. Steroids and immunosuppressants are the standard of care for this, and it is a severe disease if not treated.
This is something that has never occured before in known medicine, so to expect them to have figured it out entirely before initiating a treatment is unreasonable. Her initial symptoms were in January of 2021, final diagnosis was made mid 2022. Would you expect your doctor to hold off on any treatment for a year and a half?