Our daughter’s mom is in prison and doesn’t want us to know. How do we handle it?

Our youngest daughter is 2 1/2.  Birthmom initially wanted only letters & pictures, but that evolved into into visits and somewhat regular communication.  About 9 month ago B-Mom disappeared. Thanks to the internet, we were able to track her down in state prison (1.5 to 3 years).  We also learned that it was a plea deal & she knew she was going away a head of time.  She didn’t tell us about the  charges or sentence, we suspect that she was a bit embarassed/ ashamed.  Mother’s day is approaching & we aren’t sure how to handle it…Do we send a card & letter to the agency & pretend we don’t know?  Or do we send a card, ect  to the prison & express our love & support to her no matter the ciircumstance?