Well peanut butter is a higher value treat for many pups and dogs. Also, peanut butter is pretty high in fat. If they fed her a good amount she may not be felling to well trying to metabolize it. I would consider too that if they gave her too much peanut butter they may have just over fed her or given her other foods that she didn't need or shouldn't have had (table scraps). I would ask. Give her a day or two to adjust to being back home and getting back to her regular diet. I wouldn't worry too much unless she is showing signs of distress. IE: vomiting, diarrhea, lethargy.
If you give in now you may very well be setting yourself up for a picky eater and a life of food struggles.
NOTE: Not to create undo worry but I would ask them to check the label on the peanut butter. Some peanut butters contain Xylitol which is very toxic to dogs and can be fatal.
And it may just be to the upset of staying at a different place at such a young age. Change in environment can cause stress when they are just getting used to being in a new home.