I may not be qualified to give you the answers, but here's what I think the sentences should be:
1. Drop 'of them'
2. I think the sentence becomes grammatically correct once you drop 'the', but it still sounds a bit awkward to me. The second half of the sentence, 'who is wearing a long dress' sounds like the speaker is there at the place where he / she can see and point finger at 'one of john's friends'. The first half, 'I like one of John's friends', seems to suggest exactly the opposite.
Correct me if I'm wrong