The service (mentioned above) from Peckham Rye to St Pancras Intl is very good. It'll take about 20 minutes or so and then you can walk another 15 mins to Euston. The bus option is also good, giving your guest the chance to at least see some of London - Aldwych, Waterloo Bridge and its views, er, Elephant and Castle, hmmmm, Walworth Road (RIP Rimworld)....