The Grindr/hookup theory would make the "friend" story more plausible... BB sounded like a pretty intelligent guy, so he has the friend take him "just in case." (But "just in case" could be... "just in case I need a cover story for my folks" or "just in case the guy flakes" or "just in case the guy ends up being a catfish." I think very rarely do people want to think "just in case my life is in danger.)
After BB goes further into the park, the friend waited for a response from BB, didn't hear back, and since (from what the family has said) BB was typically very prompt in replying to texts, the friend assumes BB was in the middle of the hookup and so the friend left. The speculation being that BB had told him he'd just get a ride back with the hookup.
And then flash forward to a few hours later, the friend still hasn't heard from BB that the hookup went fine, THEN the friend starts to panic and goes back to look for him.
I could believe that, if the friend was lying about "not knowing who the third person was" if he wanted to "protect" BB from the disapproval of BB going to a park for a hookup.
Otherwise, the friend's story sounds super sketchy. If it were a drug deal, I don't see any scenario in which a friend would just LEAVE.
Plus it would make sense why BB didn't bring his wallet or glasses. You wouldn't bring your wallet in case you encounter a thief. And you don't wear your glasses because you don't want to look "unsexy." Again, while he might have been smart enough to bring a friend in case things didn't pan out, I'd say very rarely do people assume there's the possibility their life will be in danger. Just their wallet.