If Jeremy is innocent, then it could be that Brett is just a bit of a rat and an attention-seeker who likes making things up. It could also be that he's made up the diamonds story as a way of appearing to absolve himself by making it look like Jeremy was already of a criminal frame of mind prior to meeting him, rather than him being the criminal influence on Jeremy - if you see what I mean.
On the other hand, if Jeremy is guilty, then the question that arises in my mind is how much Julie and Brett were involved. Both avenues are very interesting. If Brett was involved, then he has just opened up a potential line of inquiry by mentioning the New York lawyer. Is that a lie? Looking at it logically/analytically, it seems very unlikely that somebody would lie to that extent, due to the risk, but people don't always behave rationally. People of the 'Brett type' do sometimes tell unnecessary lies, even though it's very risky for them.
In the case of Julie, it's always assumed - even by a lot of pro-Jeremy/innocent camp types, that Jeremy corrupted her. Nobody ever, or rarely ever, considers the possibility that it was more of a two-way relationship, or even that Julie was the one who corrupted Jeremy, not the other way round.