Facebook Messenger for iPhone: How to hide active status?
How do you know your friend was using the iOS app (this may be a question for Web Apps)?
A blue circle with the white check means it was sent successfully, doesn't mean your friend was active or even read your message.
If you go to facebook.com/messages or messenger.com and navigate to the settings, you can toggle your online/offline status. Not sure how this carries over to the iPhone app showing active/not active. Gear Icon (top left) → Settings → Account → Green Toggle.


Asking your friend for more detail would be helpful.