Must be worrying for you not knowing the history, and not wanting to upset a new foster by going back and forth to the vets.
Is the Whiskas Complete a dry food? If so I'd probably go the other way, cut out the dry and leave the wet. First off it will help to hydrate her and secondly you are keeping the nutritious part of the diet (animal protein) and losing all the cereals which are superfluous and some cats struggle to process. Noah went from three soft stinkers a day
to one formed poop when I cut out the grains and Mark is getting similar results with Kylie. The other thing that could be worth trying are some probiotics (friendly bacteria) such as LactoB, any bout of diarrhoea can cause an imbalance between good and bad guys, plus the gut flora form part of the immune system.