I will never understand why prosecutors do not request a delay in the sentencing of those who plea bargain with proffered testimony until the testimony is given in open court.
I had to read the article twice, but the mother's trial (and I use the term "mother" very loosely in this case) started on Monday. I doubt if the trial will last more than a week or two. The guy isn't going to be sentenced for two months. If he doesn't keep up his side of the plea bargain--that he testifies truthfully--the prosecutor will revoke the plea bargain agreement and his case goes to trial.
And just because the defendant and the prosecutor have a plea agreement on sentencing doesn't mean the judge automatically goes along with their agreement. They do about 75% of the time, and in that other 25%, the judge usually sentences over what the plea agreement is (I'd say about 20%), with the other 5% being under the plea agreement.