[aigel]

I would take no chances and crate / kennel the dog and have it on a leash. No free roaming where the cat could be. Do NOT assume the cat will defend itself. You will risk injury to the dog and death to the cat if you allow them to meet.

I know it is the MIL's dog, so maybe you have less sympathy for it, but killing smaller furry things is an instinct in dogs. Only if they grow up around cats etc. will you have a fair chance they won't have the desire ...

G