I think "Heesu in Class 2" would be a perfect recommendation...given that your friend is not into bl yet.
you should start with something soft I guess
http://www.mangago.me/read-manga/akai_ito_no_shikkou_yuuyo/
http://www.mangago.me/read-manga/isso_koe_ga_nakattara/
http://www.mangago.me/read-manga/goshujinsama_wa_ouji_ga_o_suki/
http://www.mangago.me/read-manga/karasugaoka_don_t_be_shy/
http://www.mangago.me/read-manga/ookami_kun_wa_kowakunai/
http://www.mangago.me/read-manga/kimi_to_no_dogfight/
http://www.mangago.me/read-manga/oko_sama_star/
http://www.mangago.me/read-manga/kasegimasen_kara/
http://www.mangago.me/read-manga/yowamushi_danshi_renai_hajimemasu/
http://www.mangago.me/read-manga/mad_cinderella/
and then your friend could walk her way to the bottomless pit of no return that is yaoi.
My friend is asking why I like Yaoi and what's the difference with Yaoi and boy-girl relationship and I don't know what to say. What should I recommend her?