iMessage with multiple Apple devices

I have noticed a problem with iMessage and was wondering if anyone can help explain it. I have a friend who is travelling outside the country with his iPhone. He also has an iPad and iMac which are at home connected to his wifi (all associated with the same apple id). When I try to text him from my iPhone, the texts are being sent as iMessages but because he has no cellular data, he is not receiving them. My phone is not recognizing the fact that he is not connected to wifi and has no cellular data and is not consequently resending the texts as SMS's. For him to receive my texts I have to turn iMessage off on my phone to force the texts to be sent as SMS's which he can receive. Is is possible that because his iMac and iPad are on wifi that this is causing my phone to send my texts only as iMessages? You would think that although his iMac and iPad both recieved a text from me sent as an iMessage that the iMessage service would recognize the fact that his iPhone did not get it and resend the text as an SMS, but this does not seem to be the case. Anyone have an explanation for this and a way around it? Thanks.