Foreshadowing may apply to this quote. Perhaps the author is attempting to set up a situation/event that doesn't seem very important at the time of reading this quote but will have a much more significant effect later in the text.
There is definitely some
figurative language in the first part of the quote (the word "sink"). This is perhaps used as a way of expressing/exploring links/relationships between characters, in this case "her" and "Emma".
High modality may also apply to this quote, particularly with the use of the word "must" twice, showing the necessity for a change in their friendship through the established strength of the words.
You could also talk about
syntax and perhaps establish a particular effect it has on the reader's understanding of the text. As mentioned above, the use of the semicolon contributes to a sentence structure that can be considered more long and complex.
I hope this helps!
