Kanojo no Tomodachi is a Manga in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.kanojonotomodachi.com fastest updating comic site. The Summary is
He got a cute girlfriend who was the first in his life, but her friend tempted him with a body that was lewder than hers… What would you do if your girlfriend’s friend pressed you like this?