Taunt icons for individual chartarcter

Companion quests are locked by affection, character level, and story line.

This can be seen by standing in a cantina and giving the companion gifts until their affection reaches a certain level. I am unsure what these levels are, but this page has a full list of quests that can be done for Khem Val: http://www.torhead.com/npc/3pcPzNI/khem-val#ends

I just recently started playing SWTOR as a F2P player. My experience so far has shown that as long as I complete all the side quests and heroics, I am usually a level or two higher than my current quests. Not much, to be sure, but it's enough to give me the edge I need, and I play a tank Jedi Guardian, solo except for the Heroics.

I don't PvP at all, so that's definitely not it. I run the Flashpoints once, mostly to see the content, but that's definitely a good way to pick up good gear if they run it multiple times when they're eligible for it. F2P players CAN do the heroics multiple times, so if they don't mind grouping, that would also be a good way to gain experience and potential gear upgrades.

If your friends are unable to beat a boss at the end of a sidequest, that means they may be focusing on the wrong aspects of a fight, or bringing the wrong companion with them to complement their playstyle.

They should also be keeping up with their equipment; quest rewards are important to keep you at the proper power level, and spend those commendations before you leave the planet entirely. Modded gear has a bit more complexity, but make sure your armoring, and hilt/barrels are the best you can get; they're what determine armor and damage.