Hate to be an advertisement, but I have used Oat based cream (the oat maker of milk does a good double cream, you can even whisk.
ok, it’s a tiny bit sweet, but with coronation chicken with apricots etc you’re not going to notice.
I have 1 occasional guest who is complete vegan and use it when they’re with us, it is surprisingly ok.
alternatovely, make a good mayo from scratch! With 2 people , 1 pouring and one whisking it doesn’t take long
aside form coronation chicken advice though , no, yanbu. My occasional visitor always brings their own “milk” and “butter” as we won’t get through whole pack, and it is so wasteful for me to buy specially for a short visit . As they’re vegan I also get them to make cakes or puds often - they’re a good cook, and it means we all have same pud/cake which are excellent but I don’t have hassle of trying to become a vegan cook which needs a fair amount of specialist ingredients and practice to do well. My vegan visitor loves cheese, can’t have it now, but brings her own specialist vegan ones. They wouldn’t dream about us not having cheese, and they are dairy free from allergic reasons,
tell her, that if she is going dairy free, she needs to go through her own acceptance that she will miss certain foods (like “real” cheese), and whilst that may take time to get used to, and be disappointing for her, it doesn’t mean you all have to feel deprived, so she doesn’t …life doesn’t work like that. Pregnant women are advised to give alcohol a miss, diabetics can’t do sweet stuff and carbs randomly, celiacs have real issues and her diet is no different. She has the issues around foods she has to avoid - not the other way round.
provide what you can, but ask her to bring a few bits to so that you are not double cooking . Or worse buying just for her, and then wasting left overs - such a waste of food and money. She has to get used to this .