This just seems like a trap. "Alcohol doesn't negatively impact my life." "Well, that's exactly what an alcoholic would say!" So, you can't rely on the person's own judgment. But you can't rely on others' judgment either. For any level of alcohol intake, from 1 beer a year to a liter of vodka a day, you can easily find someone who says it's too much and someone who says it's fine (or at least 'it depends').
If someone drinks one glass of wine a year, but their significant other gets upset about it (it's now "affecting their relationship"), is it them or their significant other that has A Problem?
I'm not a moralizing teetotaler or drug abolitionist here either. Recreational drug use is often fine, people can and in some cases maybe even should do it. But it should stop when the negative consequences reach a certain point, and it's probably impossible for an individual to have the perspective allowing them to see where they are relative to that point.
And I wouldn't trust about anyone else's view. It's either a feeling (when you have to rely on the "drug user", how they feel about it) or it is hard facts. Outsiders opinions are worthless.
Their family and friends however are not outsiders. They have hard facts about how the use affects them, which is often some of the most significant negative consequences of drug use. I don't think any single person is situated to confidently say that someone else's use isn't a problem. But in cases where it is a problem, family often have all the information necessary to know that it is.
Almost everyone who drinks has this view of their own alcohol consumption. They aren't always wrong about this, but certainly very many of them are. Even people who are very obviously experiencing negative consequences of alcohol use will confidently assert that this isn't the case.