Storage ideas?

Hang some dresser drawers on the wall for storage. They aren't deep and will provide some great storage. I would also hang something around the sink like a shower curtain to hide the drain pipes. I did mine with wide velcro years ago.

she could add wall cabinets for storage or a free standing storage unit

You could use stacked kitchen wall cabinets on the wall from floor to ceiling. They basically are only 12" deep.

A low dresser would provide lots of storage.

Can't really tell from your pics what the wall across from the tub looks like but that would seem to be the obvious place to add storage. You can get some great ideas for bathroom shelving here:

https://www.realhomes.com/design/bathroom-shelving-ideas

A long low dresser or bookcase could provide a great deal of storage for both bathroom and bedroom. You might be able to locate a used one and paint the same color as the bathroom walls so it is less noticeable.

Here's some ideas from Pinterest:

https://www.pinterest.com/plumbbay/bathroom-storage-ideas/

The bathroom certainly is quite an 'blank canvas' as to posibilities for making the space more efficient and convenient. From the photo you posted though, what strikes me is the lack of counterspace. I would purchase a wide vanity with drawers and lower cabinets, and put it on the left hand side. This would immediately give some dedicated space for supplies and towels. Instead of purchasing a vanity with an integrated sink and countertop, I would choose instead a separate piece of countertop. Doing so would maximize the counterspace and would not require upsetting or relocating the sink plumbing in its present position.

If at some point in the future the budget permits, the current sink could be removed, and replaced with the new vanity. It is quite easy to cut a hole in the countertop for a new basin and taps. Of course this scenario would cover up the windows, which may not be desired; even though they are at an awkward height presently.

You could put some floating shelfs up for storage or get canines and stick them in there

My attic is like that. It was finished off when we bought the house. On one side, drawers are built into the wall, not taking up any space. Great for storage.

Put up cabinets on the opposite wall from the tub. You could also put in some shelves on the left of the sink.

I would keep with style and add a dresser that will serve so many purpouses

She could fill the whole wall space with cabinets or shelving depending on what she’s looking to store.

I would personally suggest to make use of a low dresser or bookcase.