Make sure your contract is watertight and ask if the landlord has an inventory, if they have (often they dont as the the agency will charge for this), find out exactly what is on it and how detailed it is, just to cover you if anything breaks and needs to be replaced.
Secondly, check the flat for structural work too. I just moved out of a rented house and found a big patch of damp behind a cupboard when I moved it, went to check the exterior of the property and there's a crumbling patch of brickwork. Now obviously, this is the responsibility of the landlord to fix, but, if any major work needs to be done to it (which I reckon it will), the new tenant will have to be relocated almost certainly. This has happened to friends of mine too.
lastly, I know he/she may be a good friend, but make sure that if they do a runner then you're not going to be solely liable for any issues that may arise. I had another friend lose a sizeable deposit after a "mate" of his ruined carpets, fixtures/fittings and pee'd off whilst he was on holiday.