That makes two of us = the green stuff makes two of us?
I don't why the stuff is green? The food the man ate was green?

'He splattered,' my mohter said in her most matter-of-fact tone. She paused, then added, 'The stuff that came out of him was green. I have never forgotten it.'
That makes two of us. Mom.
"That makes two of us, Mom." => In the context this means "I have never forgotten it either."

Apparently someone vomited something that was green in color. From the context you've provided, it seems reasonable to assume that he'd eaten something that was green.

"That makes two of us."
That makes much sense to me.