I think OP said "my biggest concern is others assuming I produced those images"
I don't think OP said that the person gave her photos to the client and the client couldn't tell that she wasn't the OP.
And I think if 'she' rocked up to the client to show her photos as the official photographer given she wasn't and wasn't paid, we would be having a different sort of thread here.
The client also didn't post the images, the 'she' posted it on her own business page, so I don't know why you're so concerned about watermarked images to the client.