My best friend for 10 years just agreed to try reading bl/yaoi mangas for the first time! ...

When you say best friend of 10 years means you're in your twenties (or older), so maybe this:
http://www.mangago.zone/home/mangalist/375062/
Or a bit younger (but some in this list are still on the smuttier side, but I'm sure my taste in "smut" is pretty light compared to others here):
http://www.mangago.zone/home/mangalist/370916/

idk I feel like you need to be a weeb from the beginning to be able to jump into a bl manga with no problems, for now I suggest a straight webtoon, bl is a bit too whack for a normie to begin with ie
http://www.mangago.zone/read-manga/suddenly_became_a_princess_one_day/
https://www.webtoons.com/en/romance/where-tangents-meet/list?title_no=421
https://www.webtoons.com/en/romance/untouchable/list?title_no=79
http://www.mangago.zone/read-manga/empress_of_another_world/?av=1
http://www.mangago.zone/read-manga/honey_blood_lee_narae/?av=3
http://www.mangago.zone/read-manga/la_dolce_vita_di_adelaide/
http://www.mangago.zone/read-manga/light_shadow/
https://www.webtoons.com/en/romance/lore-olympus/list?title_no=1320
http://www.mangago.zone/read-manga/miss_not_so_sidekick/
http://www.mangago.zone/read-manga/survive_as_the_hero_s_wife/
http://www.mangago.zone/read-manga/the_monster_duchess_and_contract_princess/
http://www.mangago.zone/read-manga/this_girl_is_a_little_wild/
https://www.webtoons.com/en/romance/age-matters/list?title_no=1364
http://www.mangago.zone/read-manga/doctor_elise/
http://www.mangago.zone/read-manga/golden_waltz/
https://www.webtoons.com/en/romance/i-love-yoo/list?title_no=986
https://www.webtoons.com/en/drama/lookism/list?title_no=1049
https://www.webtoons.com/en/drama/refundhighschool/list?title_no=1360
https://www.webtoons.com/en/romance/subzero/list?title_no=1468
https://www.webtoons.com/en/drama/lumine/list?title_no=1022
http://www.mangago.zone/read-manga/monster_child/
http://www.mangago.zone/read-manga/the_man_who_cleans_up_makeup/
BUT if you are insistent on it being BL then
http://www.mangago.zone/read-manga/december_rain/?av=1
http://www.mangago.zone/read-manga/heesu_in_class_2/?av=1
http://www.mangago.zone/read-manga/he_him_us
http://www.mangago.zone/read-manga/cosmic_boy/
http://www.mangago.zone/read-manga/peach_love/
http://www.mangago.zone/read-manga/bloody_taboo/
http://www.mangago.zone/read-manga/wolf_in_the_house/?av=1
http://www.mangago.zone/read-manga/cherry_blossoms_after_winter/?av=4
http://www.mangago.zone/read-manga/king_s_maker/
http://www.mangago.zone/read-manga/jazz_for_two/?av=2
http://www.mangago.zone/read-manga/raising_a_bat/

I made sure to add a wide variety of stuff since idk what she likes lol

If she never read mangas before, maybe start with webtoons ? It'll be easier since ut's from left to right.
King's Maker will definitly please anybody. For romance, there's also For Your Love which is great, Cherry Blossoms After Winter, Hush (today), Cinema Next Door, Hollow Lovers.
For mangas:
Rouge, http://www.mangago.zone/read-manga/hatsukoi_wa_gunjou_ni_tokeru/, Kanjou Spectrum, anything by Unohana, http://www.mangago.zone/read-manga/liquor_cigarette/.
And well, I think most people read this in their debut so Hidoku Shinaide might get her to enjoy BL.

And of course, the classical Sekai ichi hatsukoi and Junjou serie. ╮(￣▽￣)╭

http://www.mangago.zone/read-manga/doushitemo_furetakunai/ The best way to get into the bl world.Yoneda Kou's work is amazing, her stories are bitter sweet and the art is gorgeous.

I hope you make her read clean shit if not your fucking weird

Also she might not be used to the manga format yet (like reading right to left) so maybe choose something a little easier to read at first, just a suggestion though

I'm sorry I didn't realize someone commented something similar already! My bad

Yeah we're on our early twenties. Thanks, I'd like to read some of these as well!

Yeah I've read them. I totally love December Rain!! She's an adult and she knows what yaoi is so i hope she'll like December Rain.

Whoa that's a big list. Even I haven't read all of them! Thanks a lot!!

Hey she knows what she's getting into! But I'll try not to give her anything that'll weird her out, I hope.

Yeah you're right, I was thinking maybe webtoons will be better to start off with, like some people suggested.

I was also thinking about webtoons but most of which i read are incomplete. Cinema next door and hush (today) are complete so I'll tell her about these two first. Btw i love cinema next door!

And if she likes Hush, there's the yaoi sequel: My last love scenario.

I haven't read Doushitemo Furetakunai but i read Yoneda Kou's NightS (collection) which was great. So I'll read it before I suggest it to her :D

How old are you? She’s 10 she DOESNT know what she’s getting into dumbass. And don’t “try” to not give her something inappropriate DONT. Period. She’s a kid

Lmao youre the dumb ass one. Theyre already in their early twenties. Also dont speak shit to people, you could just say it politely without using any degrading words

they are like early 20s...….learn to read