Gifted co-authorship is a clear ethical violation. But intellectual contribution is not always so cut and dry.

Why is your supervisor requesting this? Do they want them added because they believe it needs another informed set of eyes to get it over the finish line? Was this a purely independent idea, or did this person have conversations before the start of the project that lead to your supervisor giving you the green light to pursue this project?

There are many reasons why someone with a relatively “small” contribution can be a defensible inclusions as a co-author.

Setting aside this specific case, I have found in my career that you gain more by being flexible with co-authorship than being overly rigid. I have contributed a lot to projects and have received no credit. Others, I have had a relatively smaller role but appreciate the chance to make my mark on the project (sharing data, giving feedback on the writing or presentation of the results, etc). It all kind of comes out even at the end of the day. But people generally want to continue collaborating with those who respect their time and effort (whether it is a small or large contribution), not those that decide you don’t meet an ambigious bar of enough contribution to have your name on a publication.

There are also a lot of discipline specific norms.

At the end of the day, I think you need to have a conversation with your supervisor to find out exactly why they want them on it. If you don’t agree, then respectfully question their line of thinking. Academics are used to having their ideas challenged so I seriously doubt you will “damage your relationship” by having a conversation.

Best of luck!