Personally, I’d delete it in case relationship took a turn for the worse. I wouldn’t want to be tempted to ruin someone’s life out of jealousy or a broken heart.
I see both sides. As off-putting as it is, the intent of showing complete trust and vulnerability should not be overlooked. Maybe just ask them to dial it back to a less offensive method of demonstration.
Maybe I've just watched too many rom-coms, but is it so unrealistic that she may have been hoping he would reciprocate her twisted romantic gesture by deleting the video in front of her?