She was in Azkaban during those events. She confronted Snape about it in HBP. How did she know about it?
Voldemort told her.
Other Death Eaters could not have heard much about the Philosopher's Stone, only that Voldemort was thwarted by Harry Potter. Bellatrix was the closest to Voldemort.
'He shares everything with me!” said Bellatrix, firing up at once. “He calls me his most loyal, his most faithful —'-Bellatrix Lestrange, Half Blood Prince.
I never believed that Voldemort fully trusted Severus Snape. He only kept him around because he thought he was useful and it was obvious that he trusted Bellatrix more, even Lucius before the fiasco at the Ministry. He might have expressed his concerns about Snape's loyalty or simply narrated the whole thing to her as he did to the other Death Eaters at the graveyard since she was in Azakaban at that time.
Other Death Eaters told her.
There was apparently a vicious rumor mill among the Death Eaters. Snape addressed it in that same conversation:
"You can carry my words back to the others who whisper behind my back, and carry false tales of my treachery to the Dark Lord!" - Severus Snape, Harry Potter and the Half-Blood Prince
Belatrix, who was the most devoted of all the Death Eaters would have been clamoring for any information she could get about what happened while she was in Azkaban. The other Death Eaters would have been her best source.
The other Death Eaters would have heard from Voldemort in the graveyard on the night he returned. He referred to Snape as someone
"who I believe has left me forever" - Voldemort, Harry Potter and the Goblet of Fire
Since Snape did not return until a few hours later, there was plenty of time for Voldemort to elaborate to the Death Eaters why Snape was a traitor.
It wasn’t secret - many Death Eaters knew.
Though Bellatrix wasn’t present at the time since she was still in Azkaban, the Dark Lord tells several of the Death Eaters about his attempt to steal the Philosopher’s Stone. That means both that several Death Eaters would already know about it by the time Bellatrix escaped from Azkaban, and that the Dark Lord himself was willing to tell his followers about it, including the followers whose true loyalties he doubted - he wasn’t keeping it secret.
“But my plan failed. I did not manage to steal the Philosopher’s Stone. I was not to be assured immortal life. I was thwarted … thwarted, once again, by Harry Potter …’
Silence once more; nothing was stirring, not even the leaves on the yew tree. The Death Eaters were quite motionless, the glittering eyes in their masks fixed upon Voldemort, and upon Harry.”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
Since several people knew about it by the time she escaped from Azkaban, it’s not clear how or who told Bellatrix about the Dark Lord’s attempt to steal the Philosopher’s Stone. One of the other Death Eaters could have told her, since presumably they discuss things among themselves. Her sister’s husband Lucius was one of the Death Eaters who knew, so he could have told her. It’s also possible the Dark Lord himself told her, since he told several of the other Death Eaters, many who were less trustworthy than her. What she knew doesn’t make who could have told her any clearer - all she mentioned knowing is that Snape tried to stop the Dark Lord.
“Why did you stop the Dark Lord procuring the Philosopher’s Stone?”
- Harry Potter and the Half-Blood Prince, Chapter 2 (Spinner’s End)
This could theoretically be taken to imply that the Dark Lord himself told her, as he didn’t mention Snape stopping him to the Death Eaters in the graveyard. However, that’s not necessarily true. Since the Dark Lord was willing to discuss the attempt to steal the Philosopher’s Stone with the Death Eaters, it’s possible he also gave them more information. He may have told them about Snape’s involvement in stopping him in the time between the graveyard meeting and Bellatrix confronting Snape. Therefore, the Dark Lord wasn’t necessarily the only one who’d know, so it’s still equally possible that another Death Eater told her of Snape’s involvement in thwarting him.