1. The OP's friend will not make up any stories or do anything different from what he would have done had the OP been in Beijing that week. He will go his normal way and meet up with the OP in the same terminal he would have gone to in any case.

2. In my personal experience (I did this once, though not at LHR, to get to a lounge where there was an FT gathering) and that of everyone else I've discussed it with, the chances of being asked why the refund is being requested are zero. Airlines process thousands of refunds on refundable tickets every day: after all, most people buy them precisely because they know there is a good chance that the trip won't happen. Agents don't care why a person wants one and don't want to waste time in irrelevant chit-chat. A passenger doesn't have to give, or to have, a reason. He or she is entitled to a refund, asks for it, and gets it. That's that. I added possible cover stories in a (perhaps misguided) attempt at humor, not because there would be any need for one. (If one is needed, use the one about consoling your gerbil's widow. That will end the discussion.)