How legally binding is a healthcare POA/advance directives?
Or what can happen if the person who has agreed to function as a healthcare POA refuses to follow the advanced directives or orders procedures specifically prohibited by the person granting the POA?
Do NOT need answer fast!! 
Thanks, CedricR.
The decisions of someone with a Durable Power of Attorney trump those in a Living Will/Advance Directive when a conflict arises. The reason for this is that the main weakness of a Living Will is that’s it’s highly hypothetical. There’s simply no way for the person writing one to know under what medical circumstances it might come into play. It’s fine to say, “no extraordinary measures should be taken to save my life,” but there’s a whole lot of wiggle room, sometimes, in what “extraordinary measures” can be. Or, conversely, “do everything you can to save me”. (Living Wills arose from the Right to Die movement, but they can be affirmative as well as prohibitive.) Most of us don’t think about it in too much detail when we’re writing it, or don’t have the medical knowledge to know what that may really mean in any particular situation.
So we can appoint someone PoA, but we do so with the understanding that they then have…well…the power of an attorney. That is, they make decisions for us when we can’t communicate. Those decisions are as legally binding as if we were making them - and we can ALWAYS revoke our Living Will or DNR if we’re conscious to do so. And, more to the point, a PoA can do it in real time, based on specific information from specific doctors about our specific situation. So it’s generally considered that specific information about a specific case, and decisions made with greater knowledge, should trump a vague statement about a hypothetical (ie, a Living Will or DNR.)
So choose your PoA carefully, and have several conversations with them, at least, outlining all sorts of “what-if” scenarios. Give them the information while you’re still alive about what you want if and when, for a variety of if and when situations.
And yes, you *do *need an answer fast. One thing most people don’t realize is that you cannot assign Power of Attorney once you’re incapacitated. Even if you can still sign a sheet of paper from your hospital bed, the hospital may not accept it. Do it now, while you’re still healthy and not on painkillers and there’s no doubt that your wishes are your wishes, and not fear and pain talking.