Ive been vegan for more than a decade and most of my circle is vegan. I don't think i know of any vegans against cut flowers. The only argument i can see is that if you were cutting flowers found in the wild then you could be harming the environment which indirectly hurts the wildlife and pollinators. But i think most vegans would be ok with buying flowers. My wife and i also just grow our own and cut only a fraction of what we plant.
Btw if we're talking about Costco for chicken, they do have reasonably sized bouquets for as cheap as $10. And while that chicken will be gone in a day or two, Ive kept a Costco bouquet alive for my wife for a minimum of a week.
I mean that he has shown bad judgement and couldn't help himself from messaging his SIL in an escalation that is clearly against the bounds of his relationship with the sister. So in the case of an addict he would be addicted to the feeling of doing this type of behavior. People can get addicted to all sorts of things, behaviors or activities. In this case i think he "couldn't help himself" from making a bad decision, which does show to me on some level he could be addicted to some part of this.