To add to my theory:
A quote from the director
" Or, it could have been someone who found out who hadn’t known before. It could have been a friend, foe, a neutral party or someone who has never even met Mike. We went through the whole gamut and landed on someone I liked. It's not necessarily poetic, but we’ve seen a lot of different reactions to people discovering Mike’s secret and this is a new reaction."
This means 1. He is arrested for his secret. And 2. The person who turned him in reacted different from anyone else. So far everyone has been pissed at him for the secret.
To me that leaves the priest and Rachel's mom.
I don't think the show would make the priest I to a bad guy after Mike made huge progress with him.
On the other hand Rachel's mom is doing this because she loves her baby. And can fans really be upset at her for protecting her daughter?
A couple episodes ago we were all thinking maybe Rachel would be better off without Mike. Now her mom did that for her.
Of course it will back fire. Mike will be exonerated and their love will be stronger than ever.
Anyway with all that said. I think it was Rachel's mom. She was too much a part of the ending to not be her.