Even if they would have a free seat in Business Class -> it would have been very disruptive to move a Y-pax to C in the middle of the flight - certainly for all the C-pax who paid big Euros to sleep on this long overnight flight from SIN to FRA.
A complaint would have been lodged by the C-pax, where a sick Y-pax is placed next to.
I believe there is a policy in place to prevent this.
Did you ask at SIN to purchase an upgrade to Premium Economy?