They provide a ton of information related to claiming and verifying the ownership of places on Google Maps. It should be quite easy since both are not yet claimed.
You can lookup help or refer the owners to the relevant forum and help pages.
What @MortenCopenhagen said is fully correct, GBP community is the right place where to ask.
However I am afraid, even not being an expert on the matter, that the issue is more related to the eligibility of the businesses, because in fact they are two food trucks, even if they are categorized as restaurants.
Food trucks are mobile and thus are not allowed to have markers on the map. If a Local Guide comes across them then they should be deleted from the map as “Does not exist”.
GBP does have provisions for places that do face-to-face business with the public but which do not have a permanent location. These are referred to as service area businesses (or more precisely service area only businesses) and they cannot be added to the map by the public, Instead the owner must set it up via GBP. When doing so they must provide a permanent address, which is normally the place the business is conducted but which the public does not attend. For a food truck that might be a central kitchen or the owner’s home. They then use GBPs options to hide the address. In this way they will still show up in local searches and will be able to have reviews, photos, etc.
In this case the owner should correct the addresses and then try to claim them (while you can’t move a business just by changing the address; in this case it would be OK as it’s a correction, not a move). If that doesn’t work then they should contact GBP support.
at the owners could do is create a new listing via GBP in which they hide the address