Les can mean either "they" or "you" (plural), so A ellos/as or A ustedes is often prepended for clarity.
That was probably ¿De verdad? as in "Really?" as in a surprise. Here, really means "a lot."
Sounds fine to me. Please report it (using the flag icon) if you haven't already done so.
If context is clear, it's optional. But without it, there's ambiguity---since les could also mean ellas as well as ustedes.