I just shot a sweet 16 that I booked as a referral from a DJ friend - same situation as you (OP). Before the family contacted me my friend (the DJ) called me to give me a heads up that the client was impossible to please and looking to nickel and dime me on everything I offer. Maybe it helped knowing that in advance, but as others have said, you just need to remain professional and calm. I worked with the client on a custom package as she wasn't pleased with one of my standard packages.
Long story short, I had a great relationship with the client while my friend, the DJ, had a horrible time with them. Same client, two different ways to approach their negotiating tactics.
It all starts with getting off on the right foot.