What could she do?--Not, assuredly, act on her conviction that it was he who had sent her the necklace and straightway send it back to him: that would be to face the possibility that she had been mistaken; nay, even if the "stranger" were he and no other, it would be something too gross for her to let him know that she had divined this, and to meet him again with that recognition in their minds. 
What could she do? - Not, certainly, straightway send the necklace back to him: for she might be mistaken. But even if the "stranger" were he, it would be too gross to let him know that she had realised this, and to meet him again with that recognition in their minds. 