I know a vegan who went back to eating meat
People discontinue all sorts of things for all sorts of reasons. A single case says nothing about whether the reasons for starting were sound.
When someone who genuinely believed in veganism, tried it sincerely and lived it for a long time ends up eating meat again, that tells you something the arguments leave out. They knew all the ethical reasons and still found that their health suffered or the diet couldn't be sustained in a real life, and their firsthand experience counts for more than theory about how a diet ought to work. If committed people keep finding they can't stay vegan, that is a mark against the idea that everyone can and should be.
Plenty of people do stop, and the reasons are worth knowing: poor planning and consequent health problems, social exhaustion, pregnancy, cost, or simply a change of mind. Survey work on former vegetarians and vegans finds health and practicality cited most often, which is useful information for anyone transitioning.
What it does not do is bear on the argument. People resume smoking after quitting without that reflecting on whether quitting was advisable. One person's experience is strong evidence about their circumstances and weak evidence about anything general, and it is being offered for the second.
Treating a personal story or an isolated case as though it outweighed systematic evidence. Individual cases are real, but they do not establish general patterns.
How it applies here: A single case is offered against a general claim that would require population-level data to assess. The case is real and is doing more work than it can carry.
Introducing a point that is not relevant to the question under discussion, drawing attention away from it rather than answering it.
How it applies here: That someone stopped tells you about their circumstances, not about whether the ethical or health case holds. The two questions are being treated as one.