In a sense they're not wrong, but it depends on what is actually happening, and what the person's attitude is. It's good to pursue a lifestyle that's increasingly less dependent on animal products, even if imperfect. But is an actual progression occurring? It can often be the case, especially with dietary things, that a person will do something they believe is good once, and then treat themselves with a "cheat" day three times to that one good choice.
My change didn't happen overnight. But I approached it the same way that I did when I quit smoking: I kept track of how long I went without eating animal products. When I messed up and caved in, I would start over at 0 the very next day, and resolve to go even more days without animal products than I had done on the previous attempt.
One of the larger barriers I had to break through was an anxiety about nutrition. By that point I had a pretty firm grasp of nutritional science already, and knew that people can get all of their nutrients from plants. Consciously I knew better. But unconsciously there was still this wild fear as to whether or not I could keep living on plants only. It felt dangerous. I was going up against a lifetime of propaganda.
The last time I intentionally ate meat was some pepperoni. At that point I had gotten so used to living on plants that it didn't taste the same anymore. For one, it turned out at least for me, that after being without meat for long enough, it didn't smell the same anymore. The odor became more rotten. It didn't and doesn't matter how fresh the meat is, it all smells like putrefying carcass now. That was one thing that made the pepperoni taste off. The other was that apparently I had gotten used to having less salt in my diet, because it was a completely overpowering, disgusting salt bomb.
And something had clicked in my head by that point. As I was eating it I kept thinking, "Why am I doing this? I'm not even enjoying it. I don't need it. This isn't right." So I stopped eating it, and I haven't felt the need to consume any animal body parts ever again.
Anyway, I think where things become frustrating depends on how a person is framing their habits. If it's something like, "I'm trying, I am working on doing better," then it's understandable. But if it sounds more like the person is trying to justify eating animals or their products, and they're either talking about it in a way where they're trying to seek validation or using "militant vegans" as a strawman to criticise (see: the majority of the comments here) - that kind of makes it hard to remain diplomatic.
In cases like the latter, why are you so preoccupied with what other people think? It's not about vegans, it's about the animals. Going vegan requires going against an immense tide of social pressure, and that burden will never go away. You need to learn to think for yourself. Because when you do, you can look more objectively at how humankind treats every other species of sentient being on the planet and use your own internal moral compass to finally recognize what's right in front of your face: it is wrong to eat them. It is wrong to exploit them. What happens in factory farms and slaughterhouses is horrific. And it can never stop until we stop supporting it.
It's a hard conversation because y'all are demanding we tiptoe around a vast injustice that is urgent and actively resulting in the extreme suffering and deaths of billions every year. That's not even getting into the other issues like health problems, environmental destruction, and pandemic and zoonotic disease potential.