In reply to The New NickB:
You should be looking at something in the 1.2-1.3m x 1m range, and 9-11cm thick. Anything with a smaller area than this (eg 1x0.9m) is only any good as a second pad. Anything bigger will be overkill for now, and too heavy for her to carry.
Be aware that if she is keen, she'll be wanting a second pad within a year. One pad is pretty limiting.
In most brands, 1m is a standard width. Check whether this will fit in your boot between any wheel arches. If it won't, then it might be useful to know that being American, Metolius and Organic make tgeir pads in 3 foot (90 cm) widths.
If you want to store it folded, you need one with a folding hinge design. If you will store it somewhere that it can be kept unfolded all the time, you could consider a 'taco' design without a hinge.
Good options would be moon warrior, snap bun (taco), ocun moonwalk, metolius session, organic simple, bd impact or dropzone (hinged). There are more. You should be able to get something decent for around £140-150, although some of these would cost more.
Hth,
Si