Your tank just needs to be very vocal when the Favour is coming to her so she can let you know what CDs she has up. If she has stronger CDs up, you can be a little more frivolous with your healing and less so if she has weaker, or none, up. I don't believe you should use Virus on the first set if at all possible, and try to tell your team not to do the same. That might alleviate some of the healing required in the harder parts of the fight.

Or as some have said, get a WHM who isn't slacking.