Absolutely she should be in that room. And she should not have to petition to do so — she is a victim, she is the mother. Victims should always have the right to confront the perps. If I were her — as a mother — I would take it to the Supremes if I needed to.
They do, prior to sentencing. Every jurisdiction that I'm familiar allows for the entry of a victim impact statement prior to sentencing. Some jurisdictions allow that this statement be read aloud, in court and by the victim themselves, or by their family if the victim is deceased.
However, most jurisdictions, including federal statutory and case law, limits the participation of potential witnesses to include them being in the audience for any portion of the case that does not involve their direct testimony.
I know that's not the way it goes in TV and the movies, but that's done in furtherance of artistic and dramatic license, not factual accuracy.