car shaking after servicing

[Canadian]

My friend has a 2000 Yaris and it was running perfectly when she took it in for servicing. The service center was a Toyota service center that has excellent work in the past. They said that they replaced a steering wheel belt. When my friend drove the car the next day, the car was shaking violently, She drove for 20 km and then had to call the service shop to get the car. They said that there was nothing wrong with the car but when she drove it it continued to shake. She called the service center again and they came out but this time when the service mechanic drove with her, the car stopped shaking and has not been shaking since then. This happened yesterday and my friend is afraid to drive her car. Does anyone have any idea what may have happened?