I actually saw Toto on that tour when they came to Melbourne. It was about to start when Jeff died so Simon was forced to make a snap decision to fly over to the States and join the band.
He cancelled all the sessions he had booked and took off. Apparently everyone understood and nobody questioned his cancellations. They would've been like, "Brah, it's Toto!".
IIRC it was a tour-only arrangement initially but he fit in so well it was a no-brainer to make it permanent.