Actually, that is incorrect. If prosecution feels they have enough compelling evidence to secure a conviction, they can proceed without the body. I am not a lawyer and looked this information up online.
If BB is listening to AT and taking advice like this, she is going to end up double-screwed. It would be in her best interests and her baby's to stop listening to AT and start taking protecting her future by coming clean with everything she knows. AT clearly seems to be using her, which is obvious in the video. "We" are being hunted. "We" are innocent. "Bailey" went on the date with Sydney. "Bailey" dropped her off at a friend's house. See, even while protesting their innocence, AT is setting precedence through the wording of his video that although he is helping Bailey run from the police, he wasn't around at the end of Sydney's disappearance. By having her appear with him in videos where she obviously doesn't deny what he claims, he can now say "she agreed with me so I wasn't lying" and deny her future statements. Hopefully, once BB is out from under ATs thumb, she will begin to see things more clearly and realize that no, it isn't you against the world and ATs got your back. It is AT against the world and he does not have yours. JMO