Ah, in my haste I had forgotten that bit (about Dervish being a PrC). Perhaps I was thinking of the Scout. I had almost suggested that earlier, actually, until I recalled he had asked about PrCs.
However, the original post remains true. In order not to take a PrC penalty he must either be a race that discounts the highest level class (Human or Half-Elf) or he must have Rogue as his favored class (Halfling). Otherwise, no matter what class he takes, he will experience an XP penalty. However, even if he is Halfling, Half-elf, or Human, he will experience an XP penalty if he takes more than 3 levels in any class other than Rogue or Barbarian, as such would place his new class and his Barbarian class more than one level apart. So, if he needed to take four levels of fighter, then he would encur the XP penalty, but not if he needed only 1-3 levels - presuming he is Human, Half-elf, or Halfling, of course.
You are right about the skill hit, however. As a tombraider skills are quite important, so Fighter would perhaps not be the best route to Dervish.
All he needs to become a Dervish at this point is a few ranks in Perform Dance (which as a Rogue he can take with ease) and two feats: Combat Expertise and Weapon Focus (any slashing). As his next level is 12th, all he would need to do is either take a level of Fighter or take a level of Rogue and use his first Rogue Special for a bonus feat. Combat Expertise might even fit the feel of the character, actually, as he has a high Int. Weapon Focus would be a bit of a loss, as it doesn't really seem to fit his character, but considering the nature of the Dervish, perhaps the GM would allow a substitution of Spring Attack? This would reduce the benefit of Dervish 3 unless the GM would allow the typical Spring Attack feat gained then to be replaced with Improved Spring Attack (ie: Bounding Assault). Another option for the second feat is Weapon Finesse, as the Dervish can use a light slashing weapon. So might Combat Reflexes, for that matter, considering the feel of the Dervish PrC.
