Your friends have said they want to tell their child, though, not tell other people or create speculation? And the assumption is that they will tell their child to not tell others? I went through this with my dc when they were 7, quite a few years ago, and they didn't tell anyone else, they were old enough to keep it confidential.
I am going to explain how it can be from the other side, in case this gives you insight. I went out on a social thing with dc7 with another family, a mother with 2 children, one dc was 8 and the dc 11. The 11 year old was adopted. The relationship between the mother and 11 year old was so poor it was barely transactional, there was fear, the mother thought the child was going to get violent when they were not allowed sweets, there was no affection or closeness or chatting, all day. Because I understand how adoption can be, because I was adopted and I know a lot of people who were adopted, it didn't shock me, though it did sadden me. But my dc had up to that point only seen affection and connection in the parent/child relationship and they were shocked and disturbed. The relationship between the mother and her 9 year old was completely different, there was a lot of touching and affection and joking. My child was really upset about it all and so I explained to my dc afterwards, even though it was a "secret". DC didn't tell anyone else and has not done. This was quite a few years ago.
I am not suggesting that this is a description of your relationship with your dc, but it might be that there have been differences which have upset the other child, there might be that they have noticed things or that your dc has said things. It is not necessarily as straightforward as you have said. I wouldn't distance myself, my advice FWIW is to talk more to your friends and see if they will open up a bit more about why they want to explain things to their dc. There may be things they have not yet said to you for fear of upsetting you, but which may in fact help you and your dc.
I really feel very strongly that in relation to the situation I described it would have been much better for the family if they had not kept the "secret" and if the mother had been more open to hearing advice from other parents including about how things might be from the child's perspective. Noone was able to advocate for the child, because it was all such a secret and of course as we read on MN, no one should dare try to advise an adoptive parent.
I am not suggesting that any of this relates to your situation, as I don't know and couldn't know, but it is worth the other posters here taking on board that each adoption is different, each adopted child is different, and having blanket rules about secrecy or anything else is not always the best thing for children.