I think unfortunately guerrilla doesn't work with invisibility and walk in shadows which is a bit regrettable.
that is a fix I was expecting in the patch. :-/ The issue was motioned couple of time. Talent guerilla is placeholder now. now His only role is lure a green player to a trap of dump points into sneak. Do not take this talent never ever. (or until a proper patch comes)
Skill Sneak was bugged in original release, allowing abusing game mechanics to receive overpowered advantage. In the Enhanced edition it was fixed and tweaked, so ... well, Sneak is totally useless now. As you noticed. No, there is no other trick to make it better, it is just crap. Put even point there is the waste.
Out of combat you could sneak without a point there, good enough. Better use skill Walk in shadow thou. In combat its uselless anyway. If you are in the game begining, consider restart or replace henchman. Or you might use save game editor to get rid of Guerilla and Sneak skill.