i haven't used the f828 (personal bias against sony...lol), but i love my 8800 to bits. but therefore, i can't offer any help in terms of quality.
you are right that your friend's existing filters wont fit, although i don't think its very big an issue. a bigger issue he may encouter, is the difficulty getting 3rd party filters which would fit on the 8800's 62.5mm. when i got my 8800 the original filters weren't here yet, and i gave up the serach. i hear it is as difficult now still to get something that fits.
perhaps your friend might also want to consider the kind of mem card his current cam uses? perhaps a migration of cards could provide some savings - at the highest jpeg settings, the 8800 exhausts a 1g card in under 300pics.
yeah..go for the d-50...but remember...the f828 has the advantage of a swivel lens...so there are advantages to that....there was an occasion where I had to go low during an event shoot....luckily i was using the f828...else I would be lying on the floor if I was using the my D70..