Mashable wrote about this problem months ago: http://mashable.com/2013/09/16/imessage-problem/ A lot of my friends have complained about this problem as well. I'm surprised Apple hasn't moved faster to come up with a solution. Seems like a lawsuit waiting to happen.
They're not Apple customers anymore. It's not a problem (in that it's not effecting their customers, it IS their fault). The best way to get this fixed would probably be for iPhone users who lose the ability to message their friend to complain to Apple, not the former customer. Sad though. Apple is so good at customer service in many contexts. But sometimes they seem to decide on one of these seemingly purposeful bli…
An ex-customer who is being irreconcilably inconvenienced by your product through no fault of their own may not produce any additional revenue for you but seems pretty unnecessarily disrespectful to treat them that way.
It would be like a restaurant refusing to increase food standards because you stopped being a regular.