Buying a Rat

My 24 year old daughter had 2 pet rats. One died recently and she was really upset about it. She loves those rats as much as her cats. She keeps talking about getting another one because the one that is left seems lonely. My husband and I were talking about getting her a rat for Christmas. She would be so excited. But, we really don't know much about them. Is there anything we should know before we get one? We were thinking of looking at Petco.

What gender is the rat that she has now? If it's a male I wouldn't get another male rat to keep with it. I've found that two female rats do pretty well together and the last two that I had came from Petco. If you have any concerns about the health of their rats or how to introduce them to your daughter's current rat I would ask the people there as I've always found them to be most helpful when I had questions.