Take some sharp and high resolution photos from inside and outside of the watch with natural light and afterwards return the watch to your friend.
Tell him you need some time to decide, your wife make troubles – whatever. If he is a friend, he should accept.
If he don’t accept, he was just a seller.
The pictures you sent, I guess he did, are not good enough to come to a quick decision about authenticity or modifications. To sum up your story would rise some questions, which should be answered before spending money.