... my sister does not know alot about engins and stuff like that ...
Sounds to me like she needs a new PS pump body (including pulley) and belt. Its a relatively easy job, but the PS body is a bit pricey. They usually are sold without the reservoir, so that the old reservoir can be transferred.
Hard to believe the PS pulley came off, if thats what happened. Those things are on there real tight.
I looked at Rockauto.com, and it looks like the pumps are sold without the pulley or pulley nut either. Appears to have a spline, so there may not be a key.
Her best bet may be to get a used one from a wrecker, and have a shop willing to install it. 4-cyl pumps are different from 6-cyl pumps. On the other hand, if the pulley came off cleanly, maybe all she needs is the pulley, pulley bolt and belt. Without photos of the engine bay, we are only just guessing at the real problem.