[souffle]

Our young females usually start to 'stand' at around 5 weeks of age so it is ceratinly possible for her to be pregnant. You may have got away with with it in that males are usually slower to mature so they are less likely to have mated her.
All you can do is take care that you give her plenty good food and extra protein and keep her as stress free as possible.
I would not have bought one from a tank with wet tail in it TBH and you must keep her isolated from Dewey just in case she develops it. Watch her very closely and the first sign of anything runny take her to the vet for antibiotics.
Also selling her to a 13yr old is not good practice. Were your parents with you?
She may be a dark grey banded / silver grey banded / sable banded / golden banded as all these can look quite greyish.
Once you have had her for 16 days if no babies arrive then you can be sure she was not pregnant.
You can give her a quarter teaspoon of live yogurt once a day to try and keep her bowel healthy but no more than that. Give plenty dry hamster food and once a day a fingernail sized piece of either egg, chicken, tofu or some fruit or veg. No more than this till you see her tummy is OK. If you overdo fresh food at this stage you can get runny bottoms.
I hope she is OK and remember strict hygine when handling between the two. Keep in different rooms and keep their food / bedding etc separate for at least 3 weeks. Wash hands and wear an aprop when handling her if need be other wise change clothes.