The autograph is not too readable, but a little higher in the document the civil servant wrote:
As witnesses were present: Franz Eduard Farwerck, brother of the groom, manufacturer, old, thirty-one years, living in Hilversum and Cornelis Hellingman, lieutenant, second class, old thirty-six years, living in Vlissingen.
Hellingman actually made quite a name being lieutenant, It appears he made quite a brave decision on 14 May 1940 (2). The Germans had bombed Rotterdam and the Netherlands were ready to surrender. Hellingman ordered several ships and blast furnaces to be blown up so the occupiers wouldn’t be able to use them. After this he fled to the UK where he served as captain on a submarine.
Hellingman was born on 27 May 1894 in Amsterdam. During his sea-time he lived in Vlissingen. He passed away on 21 June 1979 in Den Haag (The Hague). His father was Jacobus Hellingen (1862-1939) and his mother Petronella Geertruida Boot (1868-1950) (3). Cornelis had a brother and a sister.
Now this is all very interesting, but what is his relation to the Farwerck family? Well, on 15 February 1921 he married Margaretha Antoinette Borrius (1899-1966) (4) who, you guessed it, was a sister of Johanna Farwerck-Borrius.
So Johanna had her brother-in-law as best man.
(1) Source for photo (accessed 15 July 2021)
(2) Story around a photo taken during the war in IJmuiden (accessed 15 July 2021)
(3) Genealogy Hellingman (accessed 15 July 2021)
(4) Geneanet, Margaretha Borrius (accessed 15 July 2021)