It's been awhile but from what I remembered, that Caro chick was getting possessive of MC-kun and was intimidated that best girl Josette here was getting close to him... so to "steal a march" on her she went ahead and had some fun with him.
So her telling Josette about it after being given back her precious case by Josette is just her gloating and just casually sliding in the knife even though she knows Josette also likes MC-kun... Caro is just, ugh
![fml :fml: :fml:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)