Well, As far as the highest levels of japanese rangers go they pretty much view three classes as the most viable subs for the ranger. However as for Which class I would start with, it would be Theif, a level 30thf is very nice to have for gil farming / NM hunting by yourself. By going thief you get that out of the way by the time you get to the class you want (in this case ranger). Below is a little blurb about late game subbing. Pro's/Con's and why's of them.
Thief: Regarded as a useless sub for EXP party. Sneak/Trick only work with melee, Sneak attack is also only double Damage when it is Subbed, instead of the 4-10x modifier that a Thf Main recieves to that skill. This is great so support the monetary requirements of the ranger, but in a farming gil money making capacity rather than a party setting. Evasion does not stand up to defender or blink. Flee, would be situationally useful for anyone if subbed, Thf does not augement your Damage or ACC like the classes below classes.
Warrior: Berserk is good for damage output, Defender is good for Emergencies, Provoke is also for emergencies but quite useless in my opinion only cause it will just get you killed, just about the only classes that die faster than you are the whm and blm. Subs add relatively low stats, but war gives most str of the choices. Berserk Gained at lvl 30/15, Defender at 50/25. Double attack does not function with a bow.
Ninja: Purely for accuracy. Dual wielding two archer's knives (+1agi +10rng acc each lvl 28 req) - The knives, for the purpose of relating to ranged Accuracy is like wearing +42 agility. 2vit=1def 2str=1atk assumed 2agi=1rng acc (unverifyable as it isn't a visible stat we can test). As far as dual wielding goes this Perk becomes accessable Very early in the rng/nin's progression compared to most other sub's pinnacle asset to the ranger. 28/14 is dual archer knives well 28/10 with under leveled sub. Blink Ninjitsu - is equal to use as Defender ever would be. Only for use in emergencies or to save whm mana (apparently that too would be an emergency of sorts).
Samurai: Rangers always have to watch the envelope when dealing damage. However, instead of riding the fringe, the RNG/Sam apparently is voraciously taunting that fringe. The only downside of this combo, is that it only truly comes into its own at lvl 60rng/30sam because of meditate (100tp / 3min) This seems to be (if your accuracy is through the roof) that you may want to level a sam to sub once you reach higher levels. I'm sure most rangers here will attest, with good +acc, our TP gain is quite fast naturally. I generally am roughly 50-75TP ahead of other melee, if my hits are not wiffing, but you do get streaks where you lag behind. If all your WS's hit, this would be quite incredible damage, there is no doubt about that.
Samurai also has Third Eye (1attack dodged refresh 1min) that is marginally useful compared to defender or blink.
Dual Wield and Archer's Knives Math: Many people also say that dual wield is bad, as far as adding delays however. A little math. Lvl 10 dual wield has the worst modifier (1.3) at 25 another modifer bonus(1.6 and at 45nin(2.0, unreachable as subjob). If you go Ninja you will be wielding 2 archer knives for the span of your lifetime, They are 11dam/195delay. Added together they are 22dam/395delay But 395divided by 1.3 yeilds a true dual wield delay of 300delay, at level 50/25 this drops again to 244(rounded). So while your melee will never do any damage you will care about it CAN augment your TP gain. I'm sure most rangers melee between bow shots, These are Free hits, even if they hit for 1, its TP for your wicked WS's.
Note: your single weapon damage seems to drop slightly as well when Dual Wielding, so perhaps 22dmg functions at around 18 in the dual wield setup (this is purely my observation, nothing based on math but our melee damage or weapon damage is irrelevant)
Keep in mind these other side effects of low delay dual wield.
You swing twice, this prevents "wiff" spikes compared to a long delay single attack weapon. Example: in 900Delay you will swing 6 times (my other observation is minimum TP gain from a successful melee attack for 1 damage even on the fastest delay weapons yeilds a minimum of 6tp per hit). A 450 Delay would swing twice. Your median hit rate is stable. Because you clock up many many attempts in a shorter time span even if your overall hit% is the same as a scythe over many attempts. Even if your damage is lower in 6 hits, than the beat of a scythes damage in 2 we as rangers don't care, we just want TP augementation from our melee. However the Scythe only gets 2 dice rolls in 900 delay, we get 6, That's very good for a Stable amount of TP generation. Also a single dagger in a non-dual wield setting clocks Less swings than 2 of the same weapon Dual wielding. Ex: 1 Archer Knife 195 delay clocks 2 swings in 390 delay, as compared to 300 delay for 2 swings when dual wielded(with only lvl 10 dual wield).
Anyways maybe that answers your friends current question (starting class, and the other potential combinations in the future)
Tyrian - Fairy Server
31thf/21rng/15nin/22war - Rng/Nin