There’s been this meme circulating recently, especially on Facebook, about people having an “open door policy”. Leaving aside that I generally don’t get involved in such viral nuisances, I’ve not engaged with this because (a) the wording felt wrong and (b) it is not new for us and should not need saying.
About 35 years ago Noreen and I made a deliberate and considered decision not to have children. Instead we said that we would always be available for our friends, their children, etc. if and when they needed us.
All our friends know this (at least they all should know this!); most have been told at some appropriate time. And as the younger ones reach an age to properly understand (generally around 15-16, but sometimes earlier), we have a quiet conversation with them about it. What we say is ad lib but generally something like:

Know that we are here if you ever need us. No parents, however good they are (and our friends really are brilliant parents) can provide everything a child needs. However open, frank and honest your relationship with your parents there will always be something you don’t want to talk them about – but something you would like to discuss with someone detached and outside the family. That is what we are here for. You can come and talk to us at any time and about anything; literally anything: boyfriend/girlfriend problems, exam worries, sexuality; if you’re in trouble and need bailing out of the police station; if you need a bed for the night; if you want to talk about life and what direction you’re going; or you just want someone to talk to. Remember, we may be geriatrics, but we’ve been there ourselves. We were students in the heady hippie days of the late-60s/early-70s so we’ve either done it or we have a friend who has. You are very unlikely to shock us – one way or another we’ve encountered most things. We don’t do judgement. And we aren’t going to tell you what you must do. We’re here to listen and to help you work through whatever is worrying you. Just pick up the phone or turn up on our doorstep (call first if you can to ensure we’re at home) at any time; day or night. And most importantly anything you tell us is in complete confidence; it will not be repeated to your parents/family/friends unless you specifically ask that it is. This is what we are here for and why we chose not to have children of our own. Here’s a card with our contact details; put it in your wallet and keep it in case you ever need it.

Yes, this is indeed a part of why we are here. Having someone detached to talk to is important. (It’s like mentoring at work: I always did it and vowed that I always would even if management told me not to.)
Over the years we’ve helped a number of friends in various stages of separation and relationship problems, medical worries, and confusion about life. We’ve even been called at 3AM by a friend about to appear in court and who expected to end up in jail (they didn’t). More than one of our friends has said to us that although they’ve never needed to take us up on this, it is a great comfort just knowing we’re there if needed. There are several friends for whom we are either “spare parents” or “spare siblings”, which is a good way of looking at all this.
This is more than just having an “open door policy” or “a friend in need”. It is about the meaning of life; being a true friend; and being ethical.