score:2
So update:
We left the UK by vehicle and had no issues getting all the way to Bosnia, and entering.
Upvote:35
The passport stamp you posted indicates that he was admitted to the UK for landside transit on 29 May until 31 May.
He needs to either leave the UK by any means of transport on or before the end of the day 31 May, or if he is unable to do so, he can file for an extension to 31 July by filling out an online form that UKVI have made available for people stuck in the UK due to COVID-19.
I would recommend that he apply for the extension immediately, even if he thinks he might be able to leave the UK via some other method, as travel plans can and do change quickly these days.