I think the major hangup for this discussion is that each of us probably have a different definition of what counts as a friend. I think there's certainly a gradient between "friendly" and "friend", but I honestly don't think the exact label matters much. For the sake of my discussion, I will consider a "friend" someone that I would want to hang out with 1-on-1.

But I think there are certain boundaries that both sides must be respectful of. For example, as a manager you need to make it clear to your employee/friend when you are communicating as a manager vs as a friend. That distinction should not be left ambiguous as that will make things potentially awkward if there is a disconnect.

As an employee, you need to respect that fact that your manager cannot tell you everything, and understanding that being their friend doesn't mean they will give you any special treatment. You should be extra careful to avoid this, as your manager will be sensitive if they notice you are trying to take advantage of the friendship in any way.

In the end, I think these boundaries can cause the friendship to not progress past a certain point if you are actively working together, mainly because it is just extra work to move past a certain level of friendship if so.