Hello,
Welcome at this forum.
I just did a Google search as there are many interest groups regarding the WWII.
Unfortunately I found that one of them - 'Vereniging Bevrijdingskinderen' (Liberation Children Association), representing and supporting children of mainly Canadian and US soldiers, ceased to exist in 2006.
However, one of the search results was a newspaper article about a Dutch liberation child that got in touch with a family detective who helped him finding the relatives of his foreign soldier father in 2022 (!).
Hopefully other visitors of this forum have useful tips to find the Dutch relatives of the person you are helping with his search.
If not, it might be an option to get in touch with the family detective, if only for advice.
Her name is Els Leijs, and this is her website Unfortunately not in English):
https://familiezoeken.nl
Unfortunately her service is not for free though and might be costly if a lot of work has to be done, like for example DNA testing.
It is also an option to submit DNA to one or more international genealogy sites with a DNA database and hope for a match.
With regards,
Belem