Living with a reactive dog is very stressful - but try to keep the positives at the front of your mind as much as you can.
If Bailey snapped but did not bite your friend, then this is a good sign that your dog has a reasonable level of self control - he could have bitten her but didn't and gave her a warning, possibly because he knew her and whatever triggered his response was less threatening because she did it.
Did the behaviourists identify the triggers that Bailey was reacting too? If he is reacting in the home to visitors when he didn't before, and he is given a clean bill of health, then it may be worth another consultation with the behaviourist to establish what has changed and what new things he is reacting too. You refer to his behaviour as "flips" - what do you mean; does he growl, snarl, lung or has he actually bitten someone without warning?
In the mean time, it would probably be a good idea to keep him away from visitors, or keep him muzzled when they visit, and when you take him out, too. I agree with your son, though - if he is not confident with Bailey, them that will transmit down the lead and make Bailey more nervous and more reactive. A no-nonsense, calm handler can make a great deal of difference to the behaviour of a reactive dog