I'm in a group chat with my girlfriends and family, for the purpose of a weekend trip we're all doing together.
I don't participate in it much because they all know one another really well and I'm a 'newcomer', I've been with gf for almost two years but long distance and haven't spent much time with many of them.
Anyway I'm a vegan. I do not preach, comment on people's choices, say anything about it unless I'm directly questioned, I've been vegan for a long time and It's just one thing about me. If anything I try not to draw attention to it as I'm not very confrontational and don't like people commenting on my food or the other things people say about it, which I've had a lot of over 20+ years!
Anyway today people have been talking about what we're doing about food on the trip and where we're eating and who's bringing what etc.
My gf commented that for the first night she's making Mexican, and that she's preparing some vegan mince for me and beef for herself, and is happy to make more if others want to share.
One member of the group responded to what she said 'Ugh Vegan that's vile!' and another member has responded with a laughing emoticon.
I'd never dream of doing this about anyone's dietary choices. I asked my gf and she said 'He's a chef so he doesn't understand' 
I am not sure whether to respond (with what, I don't know him, have met him about 3 times), or whether to ignore but I just find it ever so rude.