If she's been using it for thrush, I would be thinking she's smoking wrong if it caused an issue...
Seriously, yes, because they contain liquid paraffin, yes, they can be a risk when clothing is soaked in the stuff, as the fabric could act as a wick.
However, when they are used for washing, significantly less is left on the skin.
To avoid blocking the drains, it's a good idea to wash the affected part by hand whilst standing. Take some of the cream (not much), dampen your hand and then apply it so it melts, wiping the majority away gently with a warm clean flannel.
Obviously being aware and avoiding cross contamination, it is also brilliant for cleansing your face and neck. Massage in until it melts, hot damp flannel. Will take off the toughest, darkest, waterproof, wax based, almost indelible makeup without the any trouble and leave you skin feeling super soft and clean.
If it doesn't work for you, another good thing to try is Eucerin shower gel. That is very gentle.
We have far too much knowledge of skin creams, lotions and potions here. I didn't even know I had sodding Psoriasis until I met DP. Two years later, I flare up and I get the same bloody diagnosis.