[KJS]

Why not a bluff to try to get her to admit something?

Try "the cops called me back today and said they suspect you were involved but didn't want to say anything at the time, [or the found some fingerprints or something]."

Then you can come with "I will vouch for you but you have to let me know if you were at all involved. I need to know whether I need to lie to the cops or not so I can get my story straight. I think I can get them off your back but you have to be straight with me."

KJS