Ok, you've been asked a lot of times now whether you have invited Bob over since the boys last saw each other and haven't answered. I'm guessing because you don't feel comfortable doing so for whatever reason. That's fine but does make it harder to advise.
If you haven't, then I'd repeat the advice that you should. Yes, it would be nice if your friend had taken the initiative, notwithstanding that none of us know what's going on in her life. But she hasn't, so you should. You really can't be unhappy about your friend not doing something you're not prepared to do.
And if you have invited Bob and he hasn't come, well then I think added to the non-invitation to the party, you have your answer. And no, it wouldn't be lovely if your friend were to invite DS round regardless of Bob's desire not to pursue the friendship. It would be really awkward for all concerned. You've said you don't want Bob to feel duty bound to spend time with DS, but if he no longer wishes to be friends, then duty bound is what any social interaction between them would be.
Lastly, whatever happens with Bob, DS needs other friends and social avenues too. Scouts is a good idea. Because people move house, die, fall out, do all kinds of things. Bob might be happy to pursue the friendship now but then they have a huge row in 12 months time. Chess too, is there a chess club anywhere near you? Or other board games maybe, if he likes chess would he be interested in Battleships, Risk, Scrabble?