I think it’s important to remember that, while it still doesn’t make it justified, some people do actually at least think they have good reasons for not being vegan anymore. For example, rather than just the usual “being vegan made me ill” touted by someone not researching and following good vegan nutrition advice, someone might genuinely develop or deteriorate with a health issue/s that actually does make being vegan much more difficult, if not impossible, to be healthy with, and they may have to go back to being veggie or even eating meat again because it would be insanely difficult or even impossible for them to be vegan and not get very sick/die young. Like I said, this doesn’t make it justified, but supporting someone’s choices/actions and just understanding and respecting the choices for the reasons why they made those choices are different. Also, you can love the sinner without loving the sin. Whether you agree with someone or their behaviour or not, no matter how passionately opposed to someone’s choices you may be, I believe it’s important to still be kind, civil, compassionate and understanding towards them as a sentient being and to their issues. Don’t get me wrong, it’s very important to promote veganism and help animals and the planet, but it’s also important to be kind to those who are not vegan, especially if they have fallen on hard times with their mental/physical health.
For another example, I used to be vegan, and I keep trying. When I am vegan, I can be a very passionate vegan. I even started writing a book called “Why You Should Go Vegan”! However, I have also developed binge eating disorder and also OCD, which, combined, make it insanely difficult to go vegan and stay vegan, so I am mostly just veggie at the moment.
The best way to deal with them and get them to go vegan is to be kind, compassionate, positive, polite and civil to them, answering their questions and concerns in helpful, compassionate, understanding and kind ways, kindly supporting them to go vegan again, or to find ways in which they can go vegan again.