Commodore braking fluidity

Over the years my brother-in-law had noticed a fault code for the ABS system showing on his 2005 VE Commodore, but there was nothing abnormal until last week when the brake pedal went all the way down and he effectively had no stopping power. He had to rely on the hand brake to stop. Incidentally, he had topped up the brake fluid reservoir with power steering fluid a few days earlier, but I don't think any air would have got into the system as the reservoir fluid level was still above the MIN mark at the time he topped it up. Can you explain why this would happen?