Depends too on the time your daughter is over there. It is possible the mother doesn't allow a lot of eating of any type between meals so expects her daughter to wait as well. Serving snack is work if it means get them up the table, washed up etc. and especially if you don't want them wherever you have sent them to play ie girl's bedroom, livingroom where tv is, etc.

I would talk to the mom based on the fact the child said something to you means the mom has said something the child has at least overheard. Ask your own child if they have other food for snack while she is there or do they not eat.