I don't find it really appropriate to try and theorize this and that when we don't have the proper information, it just creates imaginary scenarios that may or may not even have roots in truth. But for the sake of this argument I'll address it;
Assuming that this individual has been accepting, but not consuming, particular non-vegan foods this whole time in an effort not to be the 'odd man out' or seem like a 'special snow flake' that does not excuse her choice in addressing the issue.
The phrase "not acceptable" is frankly, not acceptable! Her follow up demand of receiving something else is pushing the 'unacceptableness' to new heights.
If she has been bottling this up for some time that responsibility is solely on her shoulders as it was a choice she made and maintained a lie that she grew to resent. Should she have wanted to come right out with it and request that the cheesecake is no longer given to her she had plenty of more appropriate ways of doing so;"Hello Thipu, I received your gifts. That was very generous of you and I appreciate you thinking of me. As much as you know I used to love cheesecake I am afraid that it's not something I really eat anymore. I really appreciate you including me this year but in the future please save your money for someone who will enjoy the treats. How was your holiday, did you enjoy it? How is X Y Z? Oh the weather sounds lovely.."
And if she wanted to just come right out and admit to a lie she can do so gracefully:"Thinpu I know that for some time now the cheesecakes have been a tradition and I did not want to come across as rude for not accepting them but I am afraid I cannot enjoy them myself since they're outside of my dietary needs. I did not tell you sooner because I was fearful of looking rude and arrogant but I understand now that I should have been honest with you in the past. I apologize for lying, my intent was not to be malicious. I just did not want to disrupt a family tradition and make everyone upset. In the future please save your money for someone who can enjoy these treats."
If someone should inquire to what 'treats' she WOULD enjoy at this point she can offer up some suggestions:"I really enjoy pecans and walnuts. Nothing gets me in the holiday spirit like a pecan pie or (you can make vegan pecan pie! basically by substituting butter for..vegan butter )/glazed walnuts. Thank you for thinking of me Thinpu."
Regardless of if this woman was really hiding her distaste for foods she accepted or not, she was acting crass and demanding.