If you're a US citizen and the friend is an EU citizen, you may not have very long to wait.

I don't recall there being anything in the baggage claim area, where I sat down for half an hour and waited for my dad (we were on the same flight).

In any case, it is not landside - there is customs after baggage reclaim, which you bypass by stating to the officer that you just arrived from London (well, provided the UK hasn't left the EU).