Cisgender doesn’t mean trans it means the exact opposite.
It is a slur to separate people who are part of the LQBTQ+ community and people who aren’t. As it was seen as weird and disgusting (still is to a lot of people) but I believe it is a reclaimed slur to only be used by members of the community. Any information I have on the word queer was found in I thread I will link here.
https://forums.episodeinteractive.com/t/lets-talk-about-the-queer-contest/402318?u=bakedpotato
This will give you a better explanation of the word.
But basically your friend is saying you should add a character that is part of the LGBTQ+ community. I would only do this if you actually want to add the character because nobody wants fake representation. If you are do research on the community so you know what you should avoid if don’t want to p*ss people off.