As a public service announcement, and as a means of utilizing some of the $40,000 worth of graduate school learning I "invested" in years back, I'd like to add this:
Mike not only sounds like an idiot when he begins to mimic the pidgin English of his girlfriend, he's also effectively preventing her from getting any benefit out of her English language interactions with him since he is now mirroring her deficiencies in the language rather than modeling the correct way of speaking for her.
It may seem to Mike (and those in his shoes) that the broken grammar they are employing will be easier for their girlfriend to understand because the girlfriend is already using it herself, but that's not the case. It is always best practice to model correct grammar just as you would always want to use English vocabulary words appropriately and according to their correct meanings.
Simplifying one's vocabulary is often a good strategy when speaking to a non-native speaker, that's true. But using simpler words is vastly different than using the WRONG words when speaking with them. If you use English words incorrectly you will pass along that error to them and they will end up confusing others or wasting a lot of time being confused. Or you might just confuse the hell out of them (and manage to look like an idiot too) if they already know what the word is actually supposed to mean. It makes zero sense all around.
In conclusion: Adopting the non-native person's grammatical errors and then using those as an ad hoc pidgin to speak with them is NOT the equivalent of the reasonable practice of using very basic vocabulary words to make things easier for ESL speakers. It is more rightly analogous to someone using the WRONG vocabulary words entirely, which will only result in confusion and tears for everyone. Just speak clear and simple English with your ESL friends and loved ones rather than doing some weird impression of them back at them. You won't sound like an idiot and you might even improve their English in the process.