I can only add abbey road - there is a web cam - you can let friends know when you will be there and they can watch for you!!
Shakespeare's Globe isn't free but cheap!!

Diamond Geezer has many, many links and articles about things to do
http://tinyurl.com/yoqfg6

as does Ian Visits, which is really wonderful
http://tinyurl.com/yep886w

And don't forget the walking tours. Although I would imagine that if he is living with a group they will find plenty to entertain themselves.

We saw the Pearly King and Queen at Covent Garden - my husband made a small donation to their charity collection and we took some great photos. They looked amazing. Google if you are not sure what I mean. We found out they are at Covent Garden on regular dates. Free to see them and chat if you like.

Kay

What a great thread! Adding this to my London list right now.

A site that just came through my Facebook news feed .. http://www.bbc.co.uk/thingstodo

All over Europe check out Sandman's Free walking tours. Only thing they take are tips! 2-3 hours of fun walking and tour guides are college kids. What a ball! And FREE!!! Check out online.

The Londonist site would also be helpful! http://londonist.com/ They provide things to do everyday of the week.