The pet shop have told you utter rubbish I'm afraid, giving your Hamster citrus fruits will not help the situation at all, they are not supposed to have these at all.

Your Hamster will need to see the vet, he will most likely give you some Baytril, which is an antibiotic, and most likely something to replace the good bacteria in your Hamsters gut. If you don't get that try offering him or her some probiotic yogurt to help. Try and keep the Hamster drinking, dehydration is the biggest worry.

I hope they will be ok.