Depends: If she’s been learning on a piano, then a cheapy keyboard will be v. disappointing to play. But if her experience has been a cheapy keyboard then OK.
You may be able to pick up a fairly decent one second hand but anything new will be in the realms of toy rather than instrument (IMO – I don’t think I’m being a piano snob, but it is possible! My digital piano was £750 and is ‘entry level’)
If you have the space and the ability to transport, then acoustic upright pianos are very often virtually given away. This may actually be your best option to get something playable for £100.
Sorry to sound negative, but you know when that chap in the office says “I’m going to buy a bike, NOT A CHEAP ONE!……I’ve got about £150 so spend!”? Well, that.