If you prefer Android’s flexibility over iOS’s heavily walled garden, you have a few options to appease your friend’s eyes and save them from the jarring puke inducing 80’s green bubbles. First off, both of these require a MacOS computer. They work by proxying the accessibility notifications to your phone, and then using accessibility API’s for iMessage to text back. Neither of these hide the fact that you don’t have your phone number connected to iMessage.
The most difficult part of both of these is accessing them outside your home network. If you know how to port forward, you’re all set. If you don’t have WiFi at home, you might be able to remote SSH your port to a server. See http://www.zenspider.com/ruby/2011/11/ssh-tunneling-via-osx-s-launchctl.html
AirMessage
AirMessage is available on the play store and has a far simpler installation routine. It’s far better maintained. It’s one caveat is it will run on your android 24/7 giving it a substantial standby battery hit. Roughly 15% depending on your phone usage.
Pros: Easy install, available from Play Store Cons: Heavy on battery. Closed source.
WeMessage
We message was developed as a pet project by Roman Scott, he open sourced it and paid for a FireBase server to keep it up. For some reason it was taken down from the Play Store and he has moved on to other projects. https://github.com/RomanScott/weMessage
Pros: Uses firebase for negligible battery usage. Open source. Cons: Requires more advanced configuration. Not well maintained.