Going to give a generous statement of possibility:
I strongly, angrily, emotionally believe the parents did a lot wrong at the *very* beginning by not being contientous and checking in with Gabby's parents when he returned without her on the 1st. That was the first mistake.
It's possible Brian had concocted a brilliant tale about she left early and flew home to her parents blahblah so they felt no need to check, but given that she lived there, and they didn't follow-up with that and ensure she was home or check in with her parents (or her!), that is a BIG flaw, in my opinion.
If they got the lawyer day 1, then everything after the fact is also entirely their fault for not helping AT ALL to allow the situation to evolve in a proper sense. Because that means they saw the trouble, knew what happened or didn't know what happened but knew SOMETHING happened (say, for example, BL got home and was all out of sorts and wouldn't talk to them or having an "episode") and said NOTHING and alerted NO ONE and checked in with NO ONE with only their protection in mind.
But, let's say they genuinely thought Gabby was A-OK and accounted for safely until they heard from her parents on Sep 10th. Can't imagine not responding just to say "wow, we are just hearing about this, we are so sorry let us look into" yadda yadda. But they don't, they realize something is very wrong by the look on Brian's face, and they lawyer up. Phew it's bad already.
My generosity comes in with what happens now (and this is all assuming they * didn't help* him after the fact except for hiring a lawyer):
I think all the speculation about how the parents should or shouldn't be handling this right now are sometimes considered without much consideration about what their actual options are. They don't have many good options. They can't really win. They probably are staying silent because they're the most hated people in America. Their son ruined their lives, too. Maybe they don't really care what happens to him now… .it's already all done with, as far as they're concerned. He got himself into this mess and he can get himself out. They don't even know how to help get him out, they don't know where he is. They can't bring Gabby back and they don't want to be involved with what he did, they want to wipe their hands of connection with him. "He's missing" is the only thing that they're willing to do at this point. Maybe that's what they're thinking — distance themselves. And they can't say anything publicly because of their legal representation is telling them not to, and their lawyer sucks and has made them look guiltier than ever. And they're smart enough to know anything they say now, it's too late. So they say nothing and hope that Brian sorts this all out somehow. They may hope he is dead, even.
Keep in mind that this opinion is coming from a place of keeping an open mind about what the situation they're in actually looks and feels like. We don't know what's going on in their heads and while we can know that they played this wrong AT THE BEGINNING, I don't know whether we can firmly say they're playing it wrong now. They may not be playing anything, if they didn't help him, they're just in this mess and resentful and really can't do anything to help the situation except hide in their home. I'm not defending them and I don't personally believe they are that innocent or naive to the facts of the case, but if they didn't have a big part to play in a "coverup" or aiding escape, then they may just be anxiously waiting for law enforcement to find him like the rest of us.
Anyways, just wanted to say that. Because while at times I am boldly assuming they are the bad guys and criminally aiding him, other times I think they're wrapped in a terrible situation with bad legal advice and they made terrible careless and callous mistakes at the beginning not to talk to her family but that could be the end of their involvement.