Death of pet rat

Question
I have been babysitting for a week now two pet rats my friend purchased from a pet store 6 mo's ago. This is their 1st time away from their loud & attentive owners. I've made sure the water & food were full & given treats. I've only taken them out once but they are in a room that is heavily used during the day so they get talked to if they're awake. Sadly this morning I came in to find one dead. They were both okay yesterday evening. There are no scratches or sign of trauma. They had seemed happy, running on the wheel constantly at night. I had heard no signs of respiratory illness from either one but I know one had had a bout with something not long after purchased. There does seem to be some rust colored something on her wiskers & teeth and around her mouth. Could you please provide a "best guess"? I'm heartsick for my friend & her boys and would feel better if I could have some answer for her. Also, I'm concerned about the other rat. Should I get her a new playmate as she has never been by herself? Should I give her more attention? I tried to hold her but she seems pretty upset & wouldn't settle. Thanks for any help you can provide.
Lisa

Answer
Rats go through a grieving process just as we do - so yes, shower the lone rat with attention and treats when she'll accept it and encourage your friend to purchase a baby from a reputable source (not a pet store!) to be her buddy. It may be awhile before either your friend or the now-lone rat will be ready for a buddy; but the sooner the better really for the sake of bonding.

As for what happened; who knows. It could have been sudden onset heart failure or something a little more long-running. I honestly can't tell you.