I don't know what to do...

Please don't de-anon. The person involved is known by people on here, and they'd probably figure it out.

The thing is, there's a girl who I know. We don't ever talk, and we're not even friends. We've probably only spoken once.

Her boyfriend started talking to me by email. He said he had access to her email too. I'd never even met him. He was being really weird.

Then, today (many weeks after the last time I heard from him), I get an email from her. I open it, not thinking anything of it. There's no message but an attached image. My email shows the attached photo in a small image at the bottom. It was of her topless.

Now, I don't really know her. In fact, I don't think she even likes me. But, I don't know what to do.

At first I felt I had a duty to tell her- because he shouldn't have done it and it wasn't fair- whether I get on with the girl or not. But then I got an email from her saying not to open the previous email and it was her boyfriend. She begged me not to open it. However, I already had almost an hour beforehand.

I don't want to admit that I opened it... because it would be awkward. It scarred me mentally and I don't want to face her. The problem is, I can't avoid her. I see her most days. If I don't tell her- I'll feel bad. But if I do, it would be horrible.

I don't see why he sent it. We don't know each other. He knows I'm female... and she's supposed to be his girlfriend.

What do I do?