Reviews are in but incomplete. Is it preferable to send manuscript for revision, or wait till complete?
The author might well like to get on with it, but it might not be fair to them. I'm depending on your analysis that the reviews you have are poor as the deciding factor. Revising based on poor advice seems to be counterproductive for the author, but s/he has no information about how much you trust the advice they have been given. 

If your reviews to date were reliable, I'd probably suggest going ahead, but under all of these conditions, it is probably better to wait. 

I once went through a series of revisions with an editor who didn't know me or trust me (I was new). After several rounds of revision, with different reviewers, the issue in question wound up back where I'd started. It was very frustrating. 

It is best for the author if you give them a task based on information that you can trust. 



Added after the question was changed. 

Suppose you get the author started on revisions, but when the last review finally arrives it changes things fundamentally? Now what? I think the author will have a valid complaint. I think you editor rules are there for a reason. It is probably a mistake to subvert them in the name of "efficiency" that may be misplaced.