There is an old prosecutor adage that "you have to go to hell to find witnesses against the devil." In other words, most co-defendant/cooperators aren't upstanding citizens and are going to come with lots of baggage. I don't know the extent of MT's involvement in the planning and execution of this murder, and that involvement is going to impact how much her cooperation with LE reduces a potential prison sentence. Still, it is not at all unusual for prosecutors to make deals in order to get the bad guy, and what we've seen so far seems to indicate MT is cooperating with LE. IMO, LE is going to make sure it can present corroborative evidence in support of her testimony. IMO non-testimonial forensic evidence is what is going to convict FD. MT's cooperation at this stage could potentially be very helpful to LE; at trial, she may be of lesser significance as she becomes one in a long line of prosecution witnesses.