We are dog sitting a dog with fleas!

[TreeTrunkThighs]

FIL's dog arrived this afternoon to stay for a week.

I have just been giving her a rub and found two big fat fleas on her belly.

What do I need to do and how quickly?

YUCK!

buy some flea shampoo and bath the dog, also spray bedding etc ASAP. My dog is eleven years old and never had fleas until last year! i was mortified,

NO! don't waste your money on supermarket flea products they are not going to sort this. Call the vet your FIL uses and ask if you are able to get some Stronghold or Advocate spot on to use on the dog, and also get a decent household spray that includes a flea hormone product in it as well as a killing agent - such as Indorex or Acclaim. Failing that, you can get Frontline from a local pharmacy (need to know the approx weight of the dog) or from an internet pharmacy. This may well be cheaper than stuff from the vet, but I would say based on recent posts here about fleas there is some concern that there may be frontline resistance possibly ?? They should also be able to sell you good household sprays.
Unless you treat both pet and environment properly then you are going to be potentially full of these little guys really soon
Also you need FIL to treat his house before the dog goes back too!