Best duo partner ?Follow

Speaking as a Warrior, (but I feel this would be applicable to Samurai as well as both jobs are primarily melee DD's with no inherent curing abilities), my favorite job to duo with is Beastmaster. Yeah, I know, it sounds strange at first, since Beastmaster is also a melee DD with no native curing abilities, but they really make fantastic duo partners.

The Beastmaster's pet essentially becomes the party tank, while the Beastmaster and his duo partner act as DD's to help kill the target before it kills the pet. If you sub /Whm and your Samurai friend subs /Dnc, you guys will also have the survivability and utility to tackle difficult challenges and explore dangerous locations. Beyond Sneak/Invisible/Spectral Jig, a Beastmaster can charm many aggressive monsters and set them free again to erase aggro, or keep them around as "bodyguards" in case a non-charmable monster aggros: sic the pet on the monster and run.

The idea behind a Beastmaster/DD job duo is to let the Pet keep hate as much as possible, at least until your enemy is close to death, at which point you both unload to take off the last of the monster's health. In situations where there are no pets to charm, you have access to jug pets that are slightly less durable but more reliable than charmed monsters. Since your friend is going to play a Samurai, he can cross the hate threshold a bit to help the pet stay alive longer or to help you handle a mischarm (when an attempt to charm a monster fails and the monster attacks you). Samurai has a decent rating in the Evasion and Parrying skills, and when he gets Saigen at level 35 he can blink or counter away enough attacks to survive until the pet re-establishes hate (which it should do unless he's spamming weapon skills). He can also use the opportunity to skill up his Archery, using Status arrows to paralyze mobs or silence spellcasters.

Rdm/Bst is an even better duo partner, since you not only have a pet to tank for you, but you have all of the curing/nuking/debuffing/enhancing spells of a Redmage. A Rdm/Bst and a Sam/Dnc would be quite formidable, and would probably never have to rest once you get Refresh and convert. However, because of the peculiar way in which Beastmaster works as a subjob (it has to be leveled as high as your main job if you plan on charming anything: charm-rate is based on your Beastmaster level even when used as a subjob, up to a maximum of your main job level), you would have to level your Beastmaster first.
The downside is that /Bst doesn't have access to jug pets (since the jugs can only be equipped on Beastmaster mainjob), so it might not be viable for certain situations where no charmable monsters are present. You also get important pet commands much later on (for example, the Leave command which lets you release a pet from charm comes at level 35 on Beastmaster mainjob, level 70 when using it as a subjob). But otherwise you can function fully as a Beastmaster and a Redmage at the same time.

Whitemage/Beastmaster is also a powerful combination, as is Dancer/Beastmaster. I can't stress enough how versatile a job Beastmaster is in a duo situation.