This is a great bit of advice. Since he's new to the MMO scene though I will play dictionary with some of the MMO jargon you used.

buffing - the ability to strengthen yourself or allies (increasing damage done, attack speed, health, power, etc)

debuffing - the ability to weaken enemies in various ways (decreasing damage done, speed of attacks, etc)

dps - damage per second.

fellowship - a group of 6 players working together.

pet class - a class which has a follower (ie pets) that helps in combat. Loremasters have animals as their pets and Captains have bannermen.

power regen - the ability to renew your power. power is used to execute most abilities

casting on the run - being able to cast your abilities while moving as opposed to having to stand still to cast.

CC - crowd control. the ability to remove an enemy from combat so that you can focus on killing his friends while he just sits there like a lump. crowd control abilities are usually broken on damage taken.

Most MMOs use a formula in group play called the holy trinity (tanking, healing, dps). Most 6 man groups generally have 1 tank, maybe 1 tank/dps (called the offtank), 1 healer, and the rest of the group is dps. This setup is mostly useful when fighting elites. Elites are monsters which are considerably stronger than regular monsters and generally take groups to bring down (although most elites in LOTRO outside of dungeons are weak enough to solo) If you want to be a good player in groups its important to understand the concept of threat and aggro.

Tanking - The ability to attract the attention (called aggro) of all nearby enemies so they attack you instead of the healer or dps classes. Threat is a behind the scenes number generated by the game that calculates how likely an enemy is to attack you. Everyone generates threat in varying degrees, but the tank has abilities specifically designed to generate it. Tanks generally sacrifice damage for survivability. The tank's job is not to kill the enemies but to distract them long enough for their mates to kill them.

Healing - Regenerates the health of the group and buffs them. Can also sometimes debuff enemies. The healer is usually not focused on the enemies but on the group.

DPS - All other damage dealing classes in the group. Their job is to kill the enemies as quickly as possible without pulling threat off of the tank. Most classes bring other utilities to the group besides dps, such as the ability to stun, debuff, or CC (crowd control). When using CC in a group keep in mind that it is generally broken on damage, so don't use area of effect abilities (AoE) anywhere near the CC.

Thats a little daunting of a concept for a completely newbie MMO player, so don't worry about it all that much when you start off. Though the idea of threat and aggro is persistent from level 1, its not really something you will have to deal with when questing, even with friends. Its more of a dungeon, skirmish, and raid concept.

Like HagbardCelineDion said, the leveling experience in LOTRO is really great, especially for a Tolkien fan. Especially if you're not forcing yourself to level as quickly as possible. Though there are a few lore breaking concepts in the game, I think they've done a great job building Middle Earth. Take your time, explore when you can, definitely join a kin and make friends with the players around your level for grouping.

Reddit has a LOTRO Forum where people can get tips and more advice 

Reddit R/lotro