Hi flip. Before I settled down to being a SAHM I had my own business renting out accommodation to backpackers in London.
It really depends what kind of accommodation she wants. It's possible to get share rooms where you turn up and are placed in a room with people you don't know, and live very cheaply. Usually there is a share kitchen and bathroom and sometimes a lounge too. These places are usually large houses. Some can indeed be grim but others are not too bad. Good place to save a lot of money. If it is classed as B & B you save because you don't pay utility bills on top and you have breakfast provided, laundry of bedlinen and there should be someone cleaning.
Some backpackers get together and rent a flat together. This is more expensive and they have to take more responsibility but usually the conditions are better and you have more freedom to play loud music and have friends to stay, etc.
There are large hostels but IME these are the worst places of all, very grim indeed.
Backpackers congregate in certain areas, Cricklewood and Willesden, Kilburn, Earls Court, Notting Hill Gate and Battersea, Camden and Kentish Town for example. So the local shop windows are a good place to look. There is a tiny shop window right next to Earls Court tube station that is crammed with cards advertising accommodation.
There used to be free weekly papers available outside some tube stations, Southern Cross and another one I don't remember the name of, these had pages of ads for backpacker accommodation.
Also if she frequents the backpacker pubs she might be able to ask around. The Spotted Dog in Willesden Green, The Black Lion in Kilburn, The Church (not sure where that is, it moves around) The Walkabout Club (I think it's in Shepherds Bush), The Polar Bear in Leicester Square.
Jobs also in the aforementioned mags. Places my tenants often used to work: Elonex, Impact Cleaners at the Nomura Bank, Macdonalds, local pubs, local temp agency, or get nannying or granny sitting jobs in The Lady. What is her normal job?