B.M.
Oooh. toughie. Yuck.
I would probably bust out with the logic and let your daughter figure it out on her own. Ask your daughter some leading questions like 'really, when does she report back to the government about her top secret activities? Does she have to go to Washington or does she just call in?" and let your daughter process it in her mind. Or even, 'really... do you think the government uses 8 year old girls as spies?'
If your daughter already knows that this friend has lied to the teacher then she has her friend somewhat figured out.
I feel bad for the poor girl - but she can't lie. She'll lose friends that way.