Must I replace my friend's lost ring by purchasing another of the same specifications, or by paying its value, or by asking her what would satisfy her?

The borrower must guarantee the borrowed item, even if they did not act negligently. The borrowed item must be guaranteed with its like, if it has one; otherwise, with its value on the day it was destroyed. Therefore, the owner of the ring must be given a similar one if found. If not found, then its value on the day it was lost. However, if its owner agrees to its value even if a similar one exists, then there is no harm.