baby rat with sore on belly

Question
My daughter found a baby rat at the pet store that is exactly what she wanted, a soft gray and white. I really like her as well.  I am guessing her to be 5 weeks old. When we held her we found an open sore on her belly.  She looks healthy other than that sore. The sore was  round and skin color, they did say it looked like she had been licking it.
They took her to the back to put medicine on it.  We checked on her the next day.  One person (at the store) said it was an umbilical hernia and that it would be fine in a while.  
 They are giving it "marmalade" (no idea if that is correct) something for wet tail and various other things they said.  

 I realize if we eventually end up with her we will need to quarantine her for awhile for the sake of the others.  Have you seen his though and do you expect her get better or this a really bad sign?  I certainly do not want to jeopardize our other rats either.   We would greatly appreciate anything you can tell us. Thank you!

Answer
You need to get that rat out of that place NOW!   Rats do NOT GET WET TAIL. They have no clue what they are doing.  Its a small abscess she has not a hernia.  I cant stand pet store employees that think they are vets.

Please get her from there ASAP.