Question

When demetrius warns that helena won't be safe alone with him in the woods at night, what is her reply?

Answer 1

When Demetrius warns that Helena won't be safe alone with him in the woods at night, she reply's, "Ay, in the temple, in the town, the field, You do me mischief. Fie, Demetrius! Your wrongs do set a scandal on my sex."

Answer 2

These are her exact words:
"Ay, in the temple, in the town, the field, You do me mischief. Fie, Demetrius! Your wrongs do set a scandal on my sex" (51) 
What Helena is trying to say is that it won't be the first time she is alone with him. This is nothing new for them - it's nothing they haven't done together before, so she is saying that there is no reason for her to be afraid of being alone with him in the woods at night.
The text we're talking about here is Shakespeare's A Midsummer Night's Dream.