In reply to AllThingsAlpine: Mile End is very good for starting out. More of a bouldering gym, but the beginers course gives you everything you nee to get bouldering/belaying and they have a good sport / indoor leading course after that. If bouldering is enough for your friend then also the Arch / The Biscuit.
Otherwise The Castle and Westway are very good as others have said. (both have a lot more routes than Mile End but a lot less bouldering)
Really depends on where in London your friend is based.