Historically accurate samurai are much different than what the American and Hollywood stereotype would have you believe.
Samurai was a conferred title and a set of commitments represented by a binding oath of loyalty, not a set of training, martial arts, tools, weapons, or skills. It could be granted and revoked. One of the most common distinctions of the title was that it was the equivalent of a license to carry weapons in public and a license to use weapons legally and lethally upon social classes lower than oneself with few to no legal repercussions.
What this means in D&D terms is that both versions of the samurai class are completely inaccurate and worthless, insofar as representing what a samurai actually was.
The question then becomes is the player desirous of playing one of those classes for some reason, or is the desire to play a samurai concept?
If it is the former, then HeyICanChan's answer has you well covered, but if it is the latter, then I have a few additional comments in addition to the excellent ones already given.
Real historically verified samurai cover a vast range of behaviors and archetypes. There are samurai who are famous for sneaking into a castle and burning the place down, or assassinating key figures. There are samurai who are famous for their horseback riding skills and those who lacked. There are those who were famous for archery skills, others for swordsmanship, and yet others famous for marksmanship (yes, guns).
There were some that were politically and socially strong, yet weak in martial prowess. There were those who were experts at torture, those that performed acts of rape and murder, who plundered and pillaged enough to make a pirate blush, and those who sought to better the lives of their peasants and were just (even by Western views). Some were artisans, and others were known for cunning or wisdom.
There are documented cases of a samurai testing a new blade upon a nearby peasant, and if it didn't cleanly cut the person in two, attacking the blade smith or refusing to pay. There are even a few stories of some who would actually swallow stones in the hope of breaking the samurai's blade.
What then is a samurai?
The key component of "being a samurai" was the oath of loyalty. This oath was an oath of absolute loyalty to a lord, to a degree that most Westerners would find uncomfortable. This oath basically placed the lord at the level of a God, and the samurai swore to obey each and every command, up to and including any morally or ethically repugnant whim the lord may take into their head, not excepting the death of loved ones or of self. Hesitation in executing any command was often interpreted as disloyalty.
In fact, this was one of the main conflicts between the versions of Christianity that arrived in early Japan and the lords of those eras - loyalty to a God was supplanting loyalty to the lords, so they saw it as a threat, and massacred thousands in an attempt to genocide any and all Christians in Japan. This was not entirely one sided as the specific sects that were trying to convert the Japanese used many morally and ethically dubious methods that were frankly against Christian teachings.
Yet it was not entirely black and white. There were ways for a samurai to remove his oath from a lord in publicly acknowledged ways, though death was often the result, unless the samurai could escape, or the lord let them go for some reason.
In fact the were many occasions where spies who were deliberately sent into the camps of enemy lords, pretended loyalty until the opportune moment. The histories also have records of great betrayals or loyalty upheld in the face of overwhelming odds.
Japan's famous ninja were all originally samurai, some of whom declined seppuku after losing a battle (often interpreted as failing to complete a lord's order which might result in the lord ordering the death of the entire family as well as the samurai). Even to this day, among the three families that claim direct descent from the ninja clans, it is indicated that among the traditional martial arts taught by ninja, a number were originally specific to samurai.
Having given this very brief and incomplete overview of samurai in history, the conclusion I would like to emphasize is that almost any base class or prestige could be used to represent the skillset of a samurai, as they were very flexible and diverse in reality.
The key factor would be the binding oath, which should ideally be role played, rather than merely represented by rules mechanics.
No mention of samurai can be made without mentioning honor. The codes of honor common to the Asian regions are foriegn to western ways of thinking and distinctly do not align with traditional concepts of chivalry.
For example, there's the very important matter of face, which is challenging for those not raised with the concept, and a source of many conflicts and misunderstandings between East and West. Face is a combination of reputation, status, honor, manners and courtesy both outgoing and incoming, and a few other more difficult concepts.
Also, an important point of note are the Ronin, wanderers without a lord. Some were because the lord had died, some because they had betrayed their lord for reasons possibly just or possibly selfish, while others had yet to find a master and were searching for a lord worthy of their loyalty. Some were considered without honor, while others were glorified due to their honorable choices.
As such, it is important to develop the characters personal code of honor and rules of face. It doesn't have to be true to the traditional Asian interpretation of honor and face. Something the player will have fun with and perhaps feel challenged with is good enough. This code determines what the character is and isn't willing to do, and what sort of lord they are looking for.
Thus, the pre-lord Ronin version of a samurai concept might fit best for your player's character: a masterless samurai searching for a lord worthy of his or her blade and skills. This could also explain why he or she is so far off the beaten path.
Disclaimer: I have lived in Japan, speak Japanese, have studied Japanese history with an emphasis on samurai and ninja, and I've even become a student of ninjutsu under two authorized teachers. So while I will not claim to be an authority figure or source I do have a better idea than most of what I'm talking about. ^^