They do have to assume but it's not always right. For example, putting a child who has lost both parents with their alcoholic uncle, just because he is family or putting a child back into an abusive family because it's better they are with family. These two examples are assuming that being with family is best but we know that in some cases it's not. We can never really know what someone else's best interest is, we can assume but it won't always be correct. Some people don't even know what is in their own best interest.
I am not sure if she thought that. I really don't think she would ever ask herself that question. I think she assumed she knew best and that I needed a tough love approach but in fact I needed the opposite. I remember threatening to quit a few times because she didn't understand me or even try, I also said I needed a gentler t, she said I needed to be challenged and to be pushed along because person centred was too gentle.