I think that the sweetest thing Riven does for Musa (from what I remember, I haven't watched S2 since it aired) is not telling anybody that he saw Musa willingly give up her powers to the scrapers... not even Musa herself at first.
He tries to give her all the time she needs to process what happened, what she did, the consequences of it, without worrying that he might reveal her secret, especially bc at this point Musa and Riven have interacted what... two or three times?
He only comes clean when the whole all thing reaches a boiling point and Musa is literally stuck. Riven earlier saw her getting aggressive on the training mats so he can at least suspect that she's not taking the whole powerless thing really well. So he doesn't leave her alone and talks to her... and to get her to open up he admits that he knows what she did.
I think that he tells her only bc he thinks it might help her to know that sombody knows the truth and does not judge her for it.
