According to Meni Rosenfeld, the DGM creator, the scores should not be adjusted if an orphan occurs. There's
a discussion in the DGM thread about this and you should raise any questions you have there. Here's what Meni says:
I can see why it is intuitive to treat orphaned block as if they never happened - thus cancelling S=S*o - but the method invariant is based on the assumption that a share has a probability of d/D to be a block. If orphans exist this is no longer the case, shares actually have a lower chance of becoming a valid block. If orphans are ignored completely, the operator will pay out more than he should (the block rewards he actually receives are lower than the method assumes). I've done the math and if you simply cancel the payment specified for the orphan blocks, everything is just right. This assumes, however, that every block, once received by the pool, has the same chance to become an orphan.
I hadn't thought of it that way. Yes, if every block has the same chance of becoming an orphan, every miner will lose exactly his fair share on average.