You know your ball is healthy and you don't know anything about the boa; that is why I would go from the ball to the boa. 6 month QT means that I would keep the boa in a different room from the ball. Of course you would feed it, water it, interact with it; but you would be careful to do that in a separate room from the ball and after you have had your ball time <tee hee!>
Behind all of this precaution for me is the threat of IBD, which can lie dormant in boas for at least 6 months, and can wipe out your entire boid collection. This is just what I would do and other members will hopefully chime in!! I hope this helps, McKinsey