"loot caves" I know the original loot cave has gone but there are others. In fact there's two in the Steppes where you first land on Earth when doing patrols. For one you need a reasonably powerful weapon and armor but if just after you spawn you turn leftt then head down those steps to an old abandoned underground station about 50m to your left then go into the first room on the right a hive night and two hive acolytes will spawn from the back/left of the room. They are each worth 25 glimmer and more importantly the 3 together are about 200 XP. Kill them, return to the surface then come back and they are there again. Not only can this build XP quite quickly it's a great way to farm glimmer too. The other one is where you spawn on patrol. Head forwards and a little left. That "central building" has steps to an underground room where 5 vandals and 3 dregs spawn. Kill them on the surface then head down the stairs. Keep going out of the back of the room where there are more steps down. Go down 2 steps and wait until your radar shows a red bar at the 3 o'clock position (as you face up the stairs). That shows the vandals/dregs have just spawned again. Run up the stairs and into the room and either clear them with a grenade or your super or a fusion rifle (a powerful one will kill 3 or 4 at a time). When they are dead just go back down the two steps and wait. Keep repeating. If you hear a "ship arrive" on the surface run up the stairs to outside and kill those dregs/vandals/shanks in the "far corner" ahead. Then return to what you were doing. Each cycle is well over 200 XP and you can do one every minute or two. In the process (like a true loot cave) you will get endless ammo drops and from time to time green or blue engrams.

Dont even waste time trying to help her get gear. Just grind to 20 so the gear she will get will count towards her light level and not count for nothing.

I have just run a friend through all the story missions up to black garden using only the story rewards and gold chest loot. Although at times it was difficult it wasn't impossible. He is now doing strikes for the last few levels to 20. I believe he was even using a level 8 auto rifle for the whole time. I even think you get bonus xp and a chance at better gear on the first run of each strike - just don't quote me on that.

If your friend is on 360 or PS4 and wants some help, she can add me. I'll go through the missions with her and help her level up. (: my husband plays more on the PS4 so if she is on there I can just use his character and level her up. Other than that doing Strikes, making sure she gets her bounties done and following the story (if she has someone with her do it on the next hardest difficulty) In no time she'll be level 20.. Able to do more strikes and eventually get herself to the weekly heroics and stuff. ;)

Strikes are probably your best bet and hope For some good random blues from engrams or drops. Ideally though... it would be best to power through the story missions first, then grind strikes/etc. For the last few levels to get to 20. It should go fairly quick, as I'm sure you know the real grind begins at 20.

Best way I can think would be to do all the bounties in-between the story missions (used to reset after each new mission) and do the story mission on a harder setting to get more xp an invite other players to your fire team to make the mission go smoother

Do story mode on normal, always try to be at least 1 level higher than the level required (eg level 12 story- be at least lvl13). If you try to do it on hard then the damage inflicted is greatly reduced and hence takes longer. Do bounties and patrols as xp is earned quickly.