Well Bisexual is being attracted to two or more genders, with a preference. Most people believe that Bisexual means being attracted to only boys and girls, which isn't true. Bisexual people CAN be attracted to only boys and girls, but not all of them are.
Gay means a non-woman being attracted to other non-women. This can include demiboys, intersex people, men (and trans men!), and non binary people. Gay is also sometimes used as an umbrella term for many LGBTQIA+ people.
Lesbian is different, and it means non-men being attracted to other non-men. This can include women (and trans women!), intersex people, non-binary people, and demigirls (sometimes demiboys too).
Queer is commonly used as an umbrella term for people who don't identify as cisgender (someone who identifies as their AGAB) OR heterosexual (someone who is attracted to those of the opposite gender/s*x). This is just a very basic definition.
I hope this helps you! You and your friend sound like great people :D
- Dino