"if they teach the people good habits it can't be all bad."
Using that logic drags the Watchtower down to the same level as any religion that teaches people good habits. Is that what your father wants to do?
They have a host of 'proofs' like this. Never let them get away with them. Point out that other religions can legitimately make the same claims. If you can name some fitting the 'proof' go for it. e.g. "Good works"=Salvation Army, Mother Teresa etc.. "Spirit directed" = ask what religion doesn't claim to be spirit directed? Does a Catholic claiming the Pope is spirit directed make it true? How is a Jay Dub making the claim any different? "Preaching work" = radio and TV has heaps of religious programming, why doesn't the WT? The Mormons go door knocking, does that prove they are as good as the Watchtower?
"They believe the Trinity" = They believe in god as portrayed by their religious leaders, as your Dubby does. Adherance to any particular doctrine is meaningless as proof without first establishing whether, or not, those leaders that provided the doctrine are truly spirit directed. Did Holy Spirit direct the WT to teach that the Generation that saw 1914 was only one generation, when 'the truth' was that it was two overlapping generations?
The truth is that the only reason JWs are special to you is because it just happens to be the cult that you had the misfortune to be born into. Someone else's kid has been born to a towel head, does that mean it is OK for him to blow up a bus?
The idea is not for you to tell them that the WT isn't special, but to ask the right questions to get them to tell you that it is not special, or that their 'proof' was faulty.
Cheers
Chris