EDIT: Sorry, I didn't know Nighten had made more than one text tool! This might work, or it might not. Sorry if it's the latter!
There is a block of code you can add an "elif" statement in:
if d.current:
if d.who == MC_Name:
at message_appear(1)
else:
at message_appear(-1)
Change it from that to this:
if d.current:
if d.who == MC_Name:
at message_appear(1)
elif d.who == [insert Friend B name]:
at message_appear(1)
else:
at message_appear(-1)
Hopefully that will work! You probably moved on from the project ages ago but I still thought I should reply. :)