I would think that the dosage wouldn't be correct. I wouldn't use dog meds on cats but the other way around, I would think it would either work not do anything at all because it wouldn't be a big enough dose, I really don't think it would overdose the dog.
Never heard of doing it, but if the dog is sensitive or has very short fur/skin, then using the cat version might be a good idea. I'm thinking of dogs like greyhounds/whippets (with sensitive skin), or the hairless dogs; along those lines.
If it is Frontline or Advantage/Advantix you have to use the one specified for the species. If the cat flea prevention can be used on dogs then it would be boxed together and say cats and dogs, but Frontlne and the other flea meds i mentioned dont, so please make sure your friend uses the right one, not one for another species.
ive heard of rescues and cat owners with lots of cats using the large dog advantage on their cats to save on costs. One of my good friends back in MS did this with her 10 indoor cats. ( they went out onleashes on weekends and fleas are horrendous in MS year round)
frontline-- i cant say ive heard about it being used like that though.
Dont know much ofanything about using revolution-- i wouldnt bc its supposed to treat soo many things (worms, heartworms, fleas, ticks etc)