By fan belt I mean V-belt, which covers all of them; serpentine, alternator, water pump, A/C. power steering etc. I used an old term, perhaps that was confusing.
My point was it wasn’t slipping when the OP went to Midas and is only an issue as a result of their “service”. In replacing the starter and battery my guess is they managed to change the belt tension, making it/them slip. If she had been using the car all along with no issues, Midas changed the oil and now it won’t start and needs a starter AND battery, and the belts are screeching??? It’s possible but I’d want the old parts so I could get them inspected. I’d also start a letter writing campaign.