PM me if you can't get it figured out. I'll ask one of our engineers on Monday at work and they'll know. We develop apps for iOS and Android. Someone on the iOS or Android team will know what's up so don't rule out that it could be something from the Android end and not your client.
I'm not an engineer, but I am Google Fu'd out for today and need a drink. This Google result page is specific to Samsung devices: One last try
So good to know for sure if it is only Samsung Androids, or all Androids. I know you said specifically Samsung in the OP but need to know to diagnose. Good luck!
If your on wifi only you can only send messages to other apple products. Phone to iPad or Apple computers. You can't send any text via iMessage to androids system. If you download a texting app and your recipient has the same app you could.