He was going to torture Bond then most likely kill him. Right? Why would he lie?
To add to the torture - on top of the physical it plays with Bond's mind, at a time when he needs to have his mind clear, fully concentrating on resisting the torture, the seeds of doubt/betrayal/paranoia can't help.
Good point. Also, I suppose there is the chance that had Bond cooperated, Le Chiffre, however unlikely it would seem on the surface, MAY have let him go free and rather than have Bond wonder about Vesper's loyalty, instead cast doubt on Mathis.
I really hope that Mathis is not a bad guy. That would be blasphemous--almost on the level of the Mission: Impossible film making Jim Phelps the villain.

Mathis is a likeable chap and is Bond's best friend in Europe. It doesn't make sense to make him a bad guy and basically sticks it to the fans if EON does so. Just try and imagine Kerim Bey or Enrico Colombo being converted to double agents in their respective films.

Oh the horror! Anyway, I, for one, will be greatly upset if Mathis winds up being a villain.
So, having said all that, I still think--and would be willing to bet--that Mathis is a good guy and that Vesper was Le Chiffre's only mole in the mission. Nevertheless, I could see where Le Chiffre either set up Mathis as was mentioned in the first paragraph or where Mathis is acting as a triple agent trying to get inside Le Chiffre's organization. Le Chiffre thinks Mathis is doubling for him when he tells Bond about his French friend, but in reality he is being used by Mathis to get information on his mysterious terrorist employers. We haven't very many double agents/triple agents in the Bond films before so I could see where EON might try to do that now in its more realistic, gritty tone of Daniel Craig film.