Yeah, fuck him. But also they should accommodate for his diet.
The reason we imprison people for life as opposed to executing them, is because we want to demonstrate compassion. Prison is merely isolation for the safety of the public.
His diet doesn’t just affect him though, if his diet is to reduce suffering of non-human amimals by not eating meat, then he’s still doing good.
Think about the flip-side of this, if in prison, vegan meals were the only option for everyone, and an inmate refused to eat until given meat.