is it safe to drive a 2009 focus if you have been advised that the motor mounts need replacing

Just don't be doing any burn-outs. Just take it easy on her until fixed.

Maybe for a short while but why not fix it?

I am not going out on a limb and tell you it perfectly safe, and then something happens and I gave bad advice, But it depends on the severity if the wear, and driving around town at 40 mph -for a while- you should be ok. But it does need to be repaired, the ribber wears and gets unstable. But if the bolt is broken then no way no how drive it.

rubber not ribber

Are you sure the motor mount(s) are bad or the vibration is not a misfire on one or more cylinders? That would cause a vibration you will feel in the steering wheel. A 6 year old car the motor mounts don't usually fail