What is your opinion of best friends

How much of your own opinion can you tell friends?

Briefly to the prehistory. Girlfriend A, single, “dated” 2 men, B and C. B gives her as much time as she needs, as far as I know they haven't had sex yet but he really likes her and wants to be with her. B knows that A is still dating C. But he doesn't know how far they'll go. In his previous relationship C was completely on his nose and not yet as far as B that he can say he wants a relationship with her. A and C sleep together and C doesn't know anything about B. I hope that was understandable now.

She says she can't decide, which can be. But I think it should be open to both. She doesn't want you to play with her, but she is no better (for me).
I asked her if she had made up her mind, whether they both now know about each other and whether she doesn't think that's a little unfair. Her justification is that she can't make up her mind and that she was hurt and now needs longer. I then told her what I thought of it. She is of the opinion that as a friend I have to be on her side and would not have my opinion to tell her. Then I made it clear again that I don't care if she dates 2 guys or 20, but that I think she should be honest with both of them. In return, she wouldn't think it would be great if one of the two was still dating someone else without her knowing ... at least she had to admit that.

So now to my real question. How much of your own opinion can a friendship take?
I think friends are there to wash your head when you screw up and when friends are not allowed to give you their opinion ... then who?
So far I've always told her what I think, that was never a problem, she's also honest with me. I suspect the problem here lies more in the fact that she knows somewhere that she is behaving unfairly, suppresses it and does not want to be pointed out by others.