I am sorry. You don't how old she is, because kids are actually good at differentiating what you eat with whom, so for example at home, mega fussy but eating everything at nursery and asking for second.
I think the attitude and expectation is often the hurdle. You present a plate with the tone of the voice and body language that expect a refusal , and the child picks it. Plus the child knows perfectly well that an alternative will be offered.
Your son could try telling your granddaughter, we don't eat that food here, but we will cook together and make something similar.
At the same time, pick your battles. In 4 days, you can't fix 27 of crappy food so he could try the " no alternatives " approach for two weekends, and if it doesn't work, accept that this is not something he will be able to change. With such limited access, he needs to focus on building a bond, and if food becomes a source of conflict, he can make compromise, so snacks will be fruits, and one meals she picks, the other picks.
Separation is tough on everyone, sorry