>>42306495
They had a falling out at some point because of what Starlight did to Mary in the past, then seeing her again prompted her to reconnect with Mary.
Glim was probably mostly surprised at Mary's motivations over her parents because as far as she knew she had none since Mary kept quiet about it when asked.
I'm not sure what Glimmy's role is in their movement, but as a puppeteer like Flawless, using Mary as the flagbearer is certainly a possibility.