Agree with Kim (gosh! That's a surprise) that the choice of network should probably be on 4G signal where she's living, at any rate if the wifi is truly dodgy.
Bobb is, I think, wrong in that there are differences in PAYG pricing, probably because the networks rely on ignorance or distress purchases. I think 3 is probably the best bet for data-heavy use (at least it was a couple of years ago), but like others I've found the giffgaff goodybags a decent balance for my usage.
If giffgaff looks sensible then it's worth getting a referrer link from an existing user - free fiver (occasionally a tenner) of non-goodybag usage for both referree and referrer.
(As a certified shill for them, obviously I'll happily supply my own link, but there are plenty of other users here - I got a link from Peli when I signed up.)