What is the ruling on a girl praying while sitting in the university library, given her distance from her residence and the unavailability of a place to pray?

You must observe prayer to the best of your ability, for example, by going to the library at a time that allows you to perform the prayer, or by leaving with ablution and praying at the nearest place. If it is difficult to find a place to pray, and going to the library is a necessity, then it is permissible for you to combine the two prayers out of need. Beware of neglecting prayer or delaying it beyond its time, for that is a great sin.