If you want to keep in touch with her, you cannot come across as strongly disapproving. However, that does not mean you have to endorse her relationship with him.
For all intents and purposes, she no longer has a relationship with him if she follows what she agreed to to get released on probation. Any further relationship within the next three years is illegal for her. Because of that, she could go back to jail soon. I expect her to violated the terms of her probation if she has not done so already.
What are her plans for life other than marrying bf who is a felon? If she has any plans, maybe you can get her moving forward on those. It may be rough since she has a felony conviction and has served time.