Hey all... I don't wanna see this get really far off track...
Tom is NOT newly gluten-free. He has been gluten free for most of his life and seems to deal with it just fine. He IS newly vegan, and that seems to have really brought out some latent SS tendencies.
Tom and Tina are cousins, but because of how they were raised (grew up next door to each other) think of each other as more like brother/sister which also might be playing into this situation.
Normally they are both great people, so this huge issue over The Cake (and I have had the cake before and it is indeed worthy of the capitalization) is out of left field and completely out of character for both of them.
I don't want to see relationships totally ruined over baked goods, but I am just not sure what to say or even if it is my place to say anything....
I will try to support Tina, and see if I can figure out a way of delicately pointing out to both of them (but especially Tom) that they are completely overreacting and blowing things way out of proportion...
My ToxicSis is newly vegan, coupled with "only organic everything is acceptable." She was already SS to begin with but her latest life-change, coupled with she seems to have joined up with a group of women who all consider themselves -- and call each other -- "goddesses" doesn't help at all (ToxicSis is the Original Female Dog). I don't deal with her and, maybe, just maybe, she'll get off her righteous path and figure out, someday, how to behave in a societally-acceptable manner (she's the one who posted negative things about our wedding menu on our FB-wedding page).
Tom...means well, I suppose, but he's only annoying those around him who actually love him (like Tina). I don't know how to recommend dealing with a person like that other than ignore (which is what I did with ToxicSis -- ultimately, she dropped out of my wedding (a relief because I wanted to kick her out anyway) and flaked (also a relief because I didn't want her to attend). My mom and OtherSis stayed out of it, as did then-DF. Essentially, I ignored her SS-ness because a) I was way too busy needing to focus on my wedding and b) ignoring is the ONLY way to stop her (if I don't respond, she runs out of steam...eventually).
Tom sounds like this and sounds like he no longer should be engaged on the topic as he will not agree with anyone or concede being wrong (according to the OP). Tina could still provide him with a GF cupcake if she wishes, but that's it (I've yet to taste GF cake that is as good as regular cake; I've had some pretty good GF chocolate chip cookies though). Otherwise, Tina should not engage the crazy right now while not cutting off Tom either.
Since Tom and Tina are so close, Tina should be able to be upfront and state, "Look, Tom, I love you and consider you my brother and admire the course you've taken to improve your lifestyle...but I'm not going to let you dictate my party dynamics to me. I want to see you at my party, but you're either going to accept the way I have it or don't come at all (but I'll be sorry not to see one of my favorite people there to share this milestone with me). This topic is no longer open for discussion. GF bean dip?"