I have not experienced it (beyond some fleeting fantasies in my head) and would never want to. But have seen it happen. For example, my first T became close friends with his former therapist and out it on the public internet. It happened when his ex-T was very old and had fast-progressing dementia and he put it in the context that he was taking care of him and wanted the world to see how great his ex-T was etc. Sure, in a disabled, terminally ill state, unable to take care of himself and even express himself! He exposed many very disturbing scenes between them, states where I doubt anyone with an intact mind and judgment would want to be seen by the whole world. If he did that with anyone in my family, I am sure he would not have escaped without serious consequences. I know many people have seen it differently but it is nowhere near my concept of friendship even if it appeared consensual.

As far as between two healthy, capable adults with reasonable judgment and decent boundaries, I do not have strong feelings against the friendship in general and think it depends on the specific situation. It is also not regulated legally in clear ways, when they try to regulate it, it's more an ethical concern than legal (except when it involves romantic/sexual relationship). The thing though is that often the people involved are not exactly psychologically healthy adults with normal friendship needs, whatever that means. It often turns into all sorts of extremes and abuse. I am sure there are many exceptions but we rarely hear about those as then they won't have a reason to talk about it publicly, it is just a normal friendship in their personal life.