The trick is to conserve your tyres and pick off the opopsition.
Keep telling Bob to reduce pace in the places where he spins the arse out most - I found it was a couple of turns before the first tunnel and the left hander between the two tunnels.
After three or four laps you'll see his tyred aren't nearly as bad as usual and you can let him pick the other drivers off one by one. Occasional use of the overtake instruction sometimes helps, such as before turn at the end of the straight.