What's "unbelievable" is that people have come up with yet another reason that a woman isn't allowed to question being left alone with her children for days while her partner spends family money on an unnecessary holiday.
I'm all about funerals. I come from a culture in which people make big efforts to attend funerals. I do it myself.
But you don't take off for three days to attend the funeral of someone you weren't close to in real life, unless you were close to a member of their family.
Funerals aren't a "plus one" event.
What's really tacky here is supporting someone in attending a funeral where they will offer no support to the mourners, but wi create obligations, and possibly expense, for them.
If one of my siblings died and the brother of one of their friends, who hadn't been close to either the sibling or any of us, showed up for an overnight stay, I would be

What he is planning is really not cool on any level. He had no business treating someone's funeral as a social occasion.