Are we sure this isn't the detective's personal interpretation? Because the witness statement in the AF doesn't mention planning a robbery at all:
"DF told Agent Folkers that he overheard a discussion between Miriam and Khari in a ceramics class at West High School shortly before school was canceled in March 2020during which Miriam discussed with Khari the idea of how they could get money. DF reported hearing Miriam tell Khari Sanford that her parents had “bands” of money and that they were rich. DF reported that “bands” likely meant thousands of dollars in cash..."
Discussing how to get money isn't quite the same thing as planning a robbery. It seems strange to me that if the witness had actually said he overheard them planning a robbery, it wouldn't be mentioned in the AF. Moreover, planning a robbery during a high school class where other people can overhear doesn't make much sense. Said conversation also took place in early March, right at the start of the period when they were both living with M's parents. Maybe they were planning on mooching of them?
Based on the evidence I've seen so far, I'm not convinced M was significantly involved. These teens aren't exactly plotters on the level of Ernst Stavro Blofeld or Richard, Duke of Gloucester. By now enough time has gone by for all of their phone conversations to have been recovered and for them to have been interrogated multiple times. If M was significantly involved, we really should've seen an arrest by now, unless the police and DA are playing some kind of 3-dimensional chess. There are also a few things that stand out to me in her text messages:
11:19:14PM-Sent a text to SD: “I hear him I think tho”
11:19:18PM-Sent a text to SD: “Like I think he just pulled up”.
11:19:20PM-Sent a text to SD: “Like w someone”.
11:19:30PM-Sent a text to SD: “But yes its him”.
11:19:30PM-Sent a text to SD: “But yes its him”.
11:22:00PM-Sent a text to SD: “It’s Huncho”
11:22:07PM-Sent a text to SD: “His friend got into a situation”.
From the above, it appears M didn't know Sanford was with Larrue (Huncho) ("like w someone" and later "It's Huncho"). Moreover, M was then told Larrue had gotten into a situation. Not "They got into a situation", but Larrue specifically. Interesting phrasing, considering that Sanford is the primary defendant. It makes me suspect they didn't tell her what actually happened. What did they tell her?