Ian Day's Answer:
It's clear that this is a serious issue. You've noticed several problems and you mention that more senior people have noticed as well. There is clearly something going on. You may have ideas as what is happening but until you raise these matters openly and honestly, you'll only have assumptions and hypothesis, nothing tangible.
Talking about sensitive matters like these is difficult at the best of times. When we are friends, we often hold back from saying things as we worry that we might hurt the feelings of our friend. However, sometimes, things just have to be said. As a friend, you will have established a relationship based on trust and respect and so you are in a perfect place to talk about these difficult things. Don't hold back or worry about damaging your relationship. What is worse: a friend hearing some difficult truths from you, or someone losing their job as these problems escalate and you stand by and watch?
It is time for you to speak your truth (openly say what you have seen and heard yourself and from other people) and enter the zone of uncomfortable debate (ZOUD). This is when conversations move from social and cosy conversations into something more meaningful and challenging. This addresses the 'elephant in the room' and faces up to what is really happening.
This will be a difficult but necessary conversation, so prepare what you will say, maybe script and practice the first few sentences of what you are going to say. Consider what is the points you want to get across and what outcome do you want to achieve. Also, carefully consider when is the best time and where is the best place for you to talk about this.
Your friend may be grateful for the opportunity to have someone to listen to them and share these issues. As you have a relationship of trust, this may be a cathartic off-loading opportunity. Be prepared to listen and give yourself plenty of time.
It's interesting that you say that for the last two years it's been a revolving door of different managers. Given this, and the problems your friend is now having, might suggest that there is something wrong with the role or the organisation. Maybe this is an impossible situation and no one would be successful in this role given the current structure, resources, timescales, etc?
This suggests that something deeper needs to change. If this is the case, your friend needs to identify what, raise the matter with key stakeholders, enter the zone of uncomfortable debate with them and identify a clear and agreed action plan.
However, it is important to understand the real root cause. It is easy to collude with other people, particularly when they are our friends. We empathise, sympathise and agree with them no matter what, and blame everyone and everything else saying things like "it's all their fault!" The best approach is to challenge and question the other person, play devil's advocate while supporting them at the same time, with the intention of really understanding. If you feel the problem is with them, be honest and let them know, if the problem is somewhere else, work on that.
Either way this person seems to be overwhelmed and needs someone to help them through this situation. You can coach them to find a solution. However, avoid taking on the burden of resolving the problem, call on support from other places such as colleagues, senior-managers, human resources and the employee assistance support if available.