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 ...
There are several things to consider.
Regular Schengen rules: No problem entering the area through another country than the Netherlands, your Dutch MVV allows and will be recognized (for this purpose) in other Schengen countries.
Covid-19 related restrictions on entry: The EU made some recommendations regarding entry in the Schengen area from outside the ...
No, you were refused entry into the Schengen Area due to the lack of a needed visa.
You did not, officially, enter the Schengen Area
You cannot be removed or deported from a country when you, officially, never entered it.
These terms apply only in cases where you:
you are illegally within (overstay or illegal entry) the country
you are considered an ...