Lol
@serenata is acting just like that girl who hated shino for her assumed reasons. Ritsuko is worried that nozomi will get trapped in a grief spiral and never move on. She is still very young, and it's very easy to get stuck in that moment forever if you aren't dragged out of it kicking and screaming, especially if you've got a reinforcing touchstone like shino to keep you stable for a long time before eventually losing that too.
In regards to "stain faithful to the dead", there is no determined period of time for this sort of thing. I know someone whose girlfriend was murdered, and wound up getting together with a mutual friend of theirs in their grief shortly after. It wasn't intentional, they just became closer due to their connections to the woman who died and found a way to support each other through their grief super fast. It's unusual, but does happen, and shouldn't be considered "callous" or "disrespectful", tbh