There is nothing wrong with this visitor eating from those banquets, even if there is some extravagance in them as you say, but she should advise the people of that household and explain to them the importance of preserving blessings, giving thanks for them and not being wasteful or extravagant in them. She should teach them to benefit from these banquets by distributing surplus food to the poor and needy; thus they will attain reward as well as celebrating their joy. 

With regard to what some people do of wasting food and throwing it away, this is wrong and is a transgression against the blessing of Allaah, and it is helping the Shaytaan. Muslim (2033) narrated that Jaabir said: I heard the Prophet (peace and blessings of Allaah be upon him) say: “The shaytaan is present with any one of you in all his affairs, and he is even present with him when he eats. If one of you drops a morsel, let him remove any dirt that has gotten onto it, then eat it, and not leave it for the shaytaan. And when he has finished let him lick his fingers, for he does not know in which part of his food the blessing is.” 

The Standing Committee was asked: What is your opinion about leftover food, and food that is surplus to one’s needs? In the college they offer different kinds of food, and the students eat a little and throw away the rest. 

They replied: Wasteful extravagance is forbidden, and wasting money is forbidden. The leftover food should be kept for another time, or given to the needy, and if there are no needy people then it should be given to animals, even if that is after drying it out, for those who can manage that. 

End quote from Fataawa al-Lajnah al-Daa’imah (22/291). 

May Allaah help us all to do that which He loves and which pleases Him. 

And Allaah knows best.