When your acquaintance came up to you and say things like “this is bad for you” as if they’re doing you a favor, like giving free advise, it’s just plain awkward.
At this point of life, I think, people already know what’s bad for them, and for their own personal reason they do it anyway. So there’s no need for telling them something is bad for them. Unless they come to you asking for advice, it’s better if you just keep it to yourself.
Unless you’re very close to them.