Seconding other posters who have said that the part of London she'll own a flat in vs the part her friends will live in, her social life will be in, and her job will be in, are likely to be very far apart. Safety-wise, this presents Issues, and night buses are no fun for anyone. On the other hand, her mortgage payments are likely to be fairly low, and so maybe she'll be able to afford taxis.
IMO the best bet, IF you can find something, would be Leyton or Leytonstone. On the central line so good into town, and rapidly filling with people exiled from Hackney. A couple of good pubs and a cycle route direct to Hackney Wick for nightlife. On the other hand, someone used to Southwest London may find it all a little gritty, and if her friends are all in Clapham it's a long way home!
I like Shooter's Hill, but it is nowhere near a tube by London standards so she'll be cabbing a lot, or catching a bus, which in that area I wouldn't want to do as a lone woman after midnight.
Croydon keeps being talked up but my god, I went there the other day and it was just as grim as I remembered.
Tottenham Hale you MIGHT still get something - but maybe not. It is on the up though.
I'd put the deposit away for year or two, myself. Once she's more settled in London and has done the partying everyone does when they first arrive and have disposable income, then maybe that'd be a better time to help - and she'll have more of an idea of what would and wouldn't be acceptable.