Or you could just tell him 'Voldemort figured out he could split his soul into pieces by killing people and then hide the pieces in either animate or inanimate objects - that diary from the 2nd book/movie was one. They've all got to be destroyed before he can be killed. There are, presumably, seven bits of Voldemort-soul lurking about, one of which is in Big V's new body. Dumbledore destroyed one - a ring with a stone in it - and located another - a locket once belonging to Slytherin - which turned out to be a fake, because someone with the initals 'R.A.B.' had already stolen it from it's oh-so-clever hiding place. Voldemort's snake is supposed to be a fourth. Then there's a cup that once belonged to Hufflepuff, and, probably, something either belonging to Ravenclaw or Gryffindor (though the only known object belonging to Gryffindor is the sword from CoS, which is soul-free). Oh, and Malfoy's supposed to kill Dumbledore, but even after he disarms him he can't do it, so Snape kills Dumbledore. And Harry and Ginny finally hook up, then break up, so Harry and the other two Musketeers can go a-hunting for horcruxes instead of finishing school.'
Possibly, that's still too detailed, but I think it covers all the bases without ruining DH by being too horribly obvious about anything. Some mention of Bill (and his injuries) and Fluer might be helpful, or the fact that Harry had Snape's old book, and had to hide it and all the deal with the Room of Requirement, but a number of people I know who read all the books didn't actually pick up on the fact that the tiara from HBP was the diadem until it was spelled out, and it most likely won't be in the movies, anyway (considering all the other stuff that was skipped), so I think you're safe skipping it as well.
Sorry for the ramble. Hope you found a decent summary, since mine obviously is wonky.
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Or you could just tell him 'Voldemort figured out he could split his soul into pieces by killing people and then hide the pieces in either animate or inanimate objects - that diary from the 2nd book/movie was one. They've all got to be destroyed before he can be killed. There are, presumably, seven bits of Voldemort-soul lurking about, one of which is in Big V's new body. Dumbledore destroyed one - a ring with a stone in it - and located another - a locket once belonging to Slytherin - which turned out to be a fake, because someone with the initals 'R.A.B.' had already stolen it from it's oh-so-clever hiding place. Voldemort's snake is supposed to be a fourth. Then there's a cup that once belonged to Hufflepuff, and, probably, something either belonging to Ravenclaw or Gryffindor (though the only known object belonging to Gryffindor is the sword from CoS, which is soul-free). Oh, and Malfoy's supposed to kill Dumbledore, but even after he disarms him he can't do it, so Snape kills Dumbledore. And Harry and Ginny finally hook up, then break up, so Harry and the other two Musketeers can go a-hunting for horcruxes instead of finishing school.'
Possibly, that's still too detailed, but I think it covers all the bases without ruining DH by being too horribly obvious about anything. Some mention of Bill (and his injuries) and Fluer might be helpful, or the fact that Harry had Snape's old book, and had to hide it and all the deal with the Room of Requirement, but a number of people I know who read all the books didn't actually pick up on the fact that the tiara from HBP was the diadem until it was spelled out, and it most likely won't be in the movies, anyway (considering all the other stuff that was skipped), so I think you're safe skipping it as well.
Sorry for the ramble. Hope you found a decent summary, since mine obviously is wonky.