I know you both, but I cannot tell you who I am, sorry. I just wanted to let you know.
Notas sobre a tradução
The story behind this text is that a group of us have a friend who is cheating on his girlfriend (who is also our friend), and we have decided to message her anonymously to let her know, as it has got pretty bad. However if we message her in English, she will suspect it was one of us that messaged her, so its best we message her in her native language (french).
This needs to be translated in the form that someone would write a casual message to a friend by text message. Also note that the 'friend' is female so would be 'Amie' as appose to 'Ami' I am guessing??