Aww! Poor pootus. I know ALL about those darned mammary tumors. Had a hairless girl, aptly named Harry, who had a good 4 surgeries to remove tumors. She kept on trucking until she was a bit over 4 years old.

As for aftercare, yes, aspen is fine. Tell the vet to imbed(subcutaneous) the sutures if he can, and use some tissue glue. They tend to really pick at them if they are exposed. She'll need to be on antibiotics for about a week so she doesn't have a reaction to the suture(it's common). If you need some Baytril, I have about 50mL on hand, and I can easily send you a cc But if the vet has injectable Baytril, you can get some of that and give it orally. It's absorbed better. Also, if the vet can use some injectable meloxicam(metacam) or buprenorphine(buprenex), it will help her recovery immensely.

As far as pain management, find some CHILDREN'S ibuprofen or acetominophen(I prefer the advil myself). Get the Children's, not the Infant's drops. She'll need about 0.2-0.3 every 4-6 hours. They like the fruit flavored, and the grape really isn't a favorite if I remember correctly.

For the first few hours, make sure she's eating and drinking, as well as staying warm. It may take a full night for her to be back to her normal, active self. If you can put a heating pad on very low, under one side of the tank, it will help alot.

For an FYI, rats, males and females, have mammary tissue ALL over their bodies. It's on their back, on their butt, on their neck, everywhere. They are extremely prone to mammary tumors, but the good news is that the majority of the tumors are benign and are easily removed as they are encapsulated(closed off from surrounding tissues).

Hope that helps! If you need anything else, let me know I can even ask my vet to talk to your vet if he has any questions. She's done plenty of my rats' surgeries, haha!