One of my players is trying to play as a rogue/druid (mostly for sneak attacking). Unfortunately, these classes don't have a lot of synergy. Are there any feats or items, ideally for pathfinder, that allow a character to increase sneak attack dice while taking levels in a class like druid? I'm hoping for something similar to Monastic Legacy (a feat which lets you advance unarmed damage dice like a monk while leveling another class), but for the sneak attack mechanic.
Complete Adventurer had a prestige class for 3.5 called the daggerspell shaper, which advanced Sneak Attack, Spells, and Wild Shape.
As for feats, I am not aware of any, but in the same vein as Daring Outlaw etc. from Complete Scoundrel, this feat seems quite reasonable:
Sneak Attack +2d6
Your druid levels stack with your rogue levels for the purposes of how much bonus damage you add on a Sneak Attack. For instance, a druid 4/rogue 3 with this feat would deal 4d6 extra damage on a Sneak Attack, as a rogue 7 would.
Your rogue levels stack with your druid levels for the purposes of Nature Bond as well as your daily uses and options for Wild Shape. For example, a druid 4/rogue 3 with this feat could have an Animal Companion with the Devotion feature, and could use Wild Shape twice per day, since the druid’s effective level would be 7 and these features are available at druid 6.
There is also the Accomplished Sneak Attacker feat that gives you an extra 1D6 to your sneak attack.
It has the limitation that it can't boost the total number of dice beyond half your character level (round up) so is only useful to multi-class characters.
Sometimes timing is everything. Paizo has just launched their Advanced Class Guide. The book is filled with Hybrid classes that mix features from different parent classes.
The book does not contain a Druid / Rogue hybrid. However, it does contain about 10 pages of material for a building a reasonable Hybrid class. It also has even more archetypes. So you may find that building your own is satisfying.
Likewise there are dozens of Druid Archetypes. If your player primarily wants Sneak Attack + Shape Change, then maybe you just keep them as a druid and cobble together an archetype that grants them a slower growing Sneak Attack.
I'm not aware of any hybrid Druid/Rogue classes. However a feat that would help is Savage Grapple from Complete Adventurer which allows you to sneak attack while grappling in wild shape. Telling blow is a feat that'd allow you to sneak attack when you critical strike (I'd presume it'd work in wild shape, confirm with DM).