I recommend "Our relationship is...". It's a manhwa but this made me read shoujo so I think it would also convince your friend that manga/manhwa are as much as fun like the other comics out there.
TBH, when it comes to het I'm not into oneshots but I prefer those inexhaustible long-chaptered ones. (￣∇￣")
I am not sure what your friends likes but the not-so-long and public-friendly ones I like are
http://www.mangago.zone/read-manga/s_l_h/
http://www.mangago.zone/read-manga/3d_kanojo/
http://www.mangago.zone/read-manga/cat_street/
I think Hiren Trip would be good it's not a long one ^^, i really loved it before i started reading yaoi ;) (sorry if i have mistakes english isn't my main langauge)
thank u guys for helping me I hope u guys give me more.
hey guys can u give me a suitable romantic manga for the begginners I wanted to show it to my friend she think manga is not fun, so i'm saying even oneshot manga is enough too and please don't give yaoi