1. Ask them of their reason as to why the Hadith is Weak. They cannot just say it’s weak without pointing to what makes it weak. Is it the chain of transmission or what?
2. Allah knowing about it doesn’t mean that we cannot grief. Ask them why do they feel sad when loose their loved ones in the family? Allah knows everything including Good or bad. Allah’s knowledge about everything doesn’t necessarily means Allah is the cause of. Human beings have Free Will and we have hands in what happens.
Allah is in charge and so He must know.
Angels have no Free will: yes and they do things according to Allah’s Command. Cursing of Muljim is one of the command and they had no choice to do otherwise.