I beleive the minimum storages is about 32 GB with about 15GB free
storage. At least this is what I have on my HP stream Tablet. ( Atom
1.8 GHz Intel Atom QuadCore processor, 1 GB of RAM, 32 GB of on board
storage plus 32 GB on a card.)
The tablet came with Windows 8.1 and I upgraded it to Windows 10 when it
came out. I had some problems installing the Aniversary edition but
when complete it worked well. When Creator came out I did not install
it until MS notified me that an upgrade for the system was available.
It went off with out a hitch and I gained 2GB of storage.
So if the phone meets the minimum requirements for the OS, then it
should install with no problems. Depending on the CPU and RAM it may
not be as fast as you desk top with an Intel i7 processor but it will be
functional.
If she has a wireless connection she can download it and install it
wireless. If she is paying by the byte for her phone connection it will
be expensive.