Nope. Friends are friends. Need not be related. And HGVC has no notion of just how we might know each other, so there is little risk. Of course, if they talk about a "rental," that's not good. But, beyond that, no coordination is needed.
I agree with both of your points, including the "where to draw the line" part. It is certainly possible, for example, that someone may have more points than they currently need on an annual basis, so they allow "friends" to stay periodically. However, they know that, within a few years, they are very likely to be using all of their points, so this is an interim thing. One scenario, anyway.
Cheers.