Firstly - Considering you are only looking after the dog for a week, I would keep them separate.
They do not need to be in the same room - A week will be over in no time.
So don't worry.
The other option is to keep the dog on lead at all times, when inside with your cats, if he is allowed to chase the cats in their own home it will traumatise them.
Not fair to your cats.
I guess it's a lesson learnt - Next time someone asks you to mind their dog...
Say no - Even if they assure (as this person did) that they don't chase cats.
Your cats should always feel safe in their own home and it seems a little unfair that you have brought someone elses dog into your house when surely he could have gone to a kennel?
If it were me, now that you are stuck with the dog - I would keep the cats separate.
The cats will want to hide away from the dog in other rooms anyway now that he has scared them.
Let them.
Things will return to normal after the dog leaves.
Good luck - But don't risk injury to your poor cats by forcing them to be together.
It would be different if it were a new dog that you owned/bought.
Assimilating them IS possible, but may take weeks and can be hard work
I wouldn't bother in this case, for such a short period of time
Note:
There is also the risk of the dog getting scatched in the eye!
Sending a dog back to the owners with one eye or a big vet bill would scare me enought to make sure they are kept separate!