Err.. I think a fighter who wants sneak attack should pay for it with the necessary levels in a class.
And he shouldn't be whining about the rogue levels, anyways; getting Evasion and Uncanny Dodge (protection from other sneak attackers!) along with +2d6 or so additional damage dice whenver you win initiative, flank someone, or (worst of all) have Improved Invisibility cast on you, is good.
As a side note, the assassin spell list is actually VERY specialized. Everything there (all 17 spells of it) is about escape, concealment, and making it easier to kill people. The Improved Invisibility spell, combined with Nondetection, is just evil.
I'm a bit dubious as to how balanced a fighter who can buy sneak attack dice with feats would be.. a human fighter that went nuts about it could pull off having +5d6 damage and Spring Attack by about 6th level. A fairly limited sort.. But unless he's an idiot and isn't paying attention to what the other PCs are, there's someone else he can coordinate flanking with, and Spring Attack will get him into position for it. Assuming a 16 STR and a +1 greatsword, monsters getting flanked and smacked around for say 7d6+5 twice or thrice a round (Haste) at 6th level isn't good.
And for the sake of throwing it up here, the Wheel of Time Noble.
The every-other-level ability (starting at 1st) of the Noble is "Call in a favor." They get a total of 10 during the course of their career, can "store" up to five at a time, and are used by making a special check (noble level + cha mod vs. a DC arbitrarily set by the DM). Favors aren't recoverable when used, but are not considered to be used up when they fail. You cannot use two favors for the same thing.
Example favors include trying to get valuable info, or trying to acquire resources without needing to pay for them. There aren't any real guidlines for them.
Inspire Confidence: +1 at 2nd level, up another plus every four levels after (caps at +5 at L18). Full-round action involving a speech, and requires a diplomacy check DC 10 + (number of people to be inspired, including noble, divided by 5). If it succeeds, the inspired gain +(#) as a competance bonus to attack rolls, skill checks, and Will saves. Duration = 10 minutes * # rounds spent on it -- max of 5 hours after 30 rounds. It can be used once a day.
Command: +2 at 4th level, +2 every fourth level past that (caps at +10 at 20th level). By passing a Charisma check = DC 15 + # of people directed, he gives that bonus in addition to the normal +2 for an Aid Another action relating to skill checks. Duration = task length, minimum of a full round. Can be used an unlimited number of times per day.
Other relevant Noble stuff: use of all simple and martial weapons, shields, and light armor. Skills = Bard list minus.. (*looks at Bard Skill List* Well, listing what skills they don't have compared to Bards would take more time and space than listing what they do have..).. Err... Anyways, Skills = Appraise, Bluff, Diplomacy, Gather Info, Innuendo, Intimidate, Knowledge (any), Listen, Perform, Ride, Sense Motive, and Speak/write Language. Points is 4 + int mod. Hit die is the d8, medium base attack bonus, moderate Reflex save (change to good if you don't feel like screwing with that..) and good will save. Lastly, at 1st, they get to pick one cross-class skill as a class skill.
Very, VERY low powered compared to 3e PC classes..