Dying Cactus [Solved]

Agree with what has been said above.

The soil that the plant is in looks pretty much like pure peat.
Not ideal for grwoing cacti in.
Make up a mix of JI No 2 or 3(70%), sharp sand(10%) and grit(20%)
And try reroot a pad in that.

When you water, make sure the water can drain away freely, and and water again when the soil has dried out completely.

 

It's recovering, I told her everything you said and she has been just watering at the sides of the pot and letting it sort of soak across or whatever the technical term is. Wahoo! Thanks all.