The quote from the "Flock" book isn't saying that it's ok for JWs to associate with their DF'd family. It's merely saying that this normally isn't a DF-ing offense.
If you are a JW and openly spending time with DF'd family members (for example, if you're seen eating with them at a restaurant), you are almost certainly going to be counseled for this. If you keep on doing it despite the counsel, the elders have 2 grounds to disfellowship you: (1) association with DF'd person; and (2) loose conduct. Remember that anything can become loose conduct if you are repeatedly counseled about it and are perceived to have a rebellious attitude toward the counsel.
So don't think for a minute that this is a "get out of jail free" card for JWs who want to hang out with DF'd family. Sure, if you keep it on the down-low and your elders aren't out to get you, then you are probably ok. But as soon as the elders decide to come after you about this, they can DF you in a heartbeat.
The best bet for avoiding judicial action for associating with DF'd family is to play the "essential family business" card. That is, if you are confronted about associating with a DF'd family member, tell the elder that it involved some important family business. Of course, that may be a bit hard to pull off if the association in question occurred at a pub at 1am...still worth a try though.