That would depend on the acidity or alkalinity of the bottled water.
How hard is her tap water? Some areas the water is ok to use on Acers.
Most of Devon and Cornwall in the south, North Yorkshire and Cumbria in the north and the western counties of Wales have soft to moderately soft water. Northumberland, Tyne & Wear and Lancashire down through the Midlands tend to have slightly hard to moderately hard water.

she lives in the west midlands ?

how much limescale does she get in her kettle? if the answer is not much then she can use tap water.

many people in East Yorks use tap water and then use a feed specifically for acers to combat our hard water.

I think she means buying bottled water from a sop.

Bottled water hardness would depend on where it was sourced. If it came from a spring in a hard water area then it would be hard, just like the tap water.

Acers not really that fussy when it comes to water, use whatever you have, tap, bottled, rainwater.