Is the underlined "that" used accurately there?
I suppose it is a relative clause, based on its meaning and function but this dialogue has been bugging me for a while since all I've learned is "that" never goes after a comma, in any cases. Is it something I'm unaware of or just an error?
If it is correct, how often is it to encounter it? Is there a meaning behind it?

Thanks a bunch for helping me out, though.
– Mister Obamium Oct 24 '21 at 14:07