Enhancement Shamans are designed to dual-wield for maximum damage. If you aren't using an offhand weapon, you won't be getting your full damage output.
The fundamental truth is that if you're looking to take the least damage, don't go with the shield -- you'll take less damage overall if you kill things faster. Less damage done means more hits taken, which means the armor bonus of the shield is moot, as you take approximately the same damage either way.
And I hope she's not trying to level on mobs 3 levels higher than her. Stick with yellow or green mobs -- they're easier, you miss them less, and, again, you'll net higher xp/hour because you'll kill them faster than you would the orange mobs.
Blood Deathknights should be spending their runes to use Heartstrike (which they get as their Blood Specialization) and Deathstrike (which is the focus of their Mastery specialization).
Talents
Here is an example talent spec: http://www.wowhead.com/talent#jcbG0srMusd
The core talents you want to be sure to pick up are Blade Barrier (reduces damage taken), Toughness (increases your armor), Bone Shield (tanking cooldown), Sanguine Fortitude (improves Icebound fortitude, a tanking cooldown), Rune Tap and Vampiric Blood (both tanking cooldowns) Improved Blood Presence (Crit immunity), and Dancing Rune Weapon (survivability AND threat cooldown).
After that, you have a choice between dipping into either Frost or Unholy. Unholy is almost always going to be strictly better, as you do not want to dual wield to tank (dual wielding has an inherently higher miss chance than 2-handed weapons, so your threat is going to be much worse) nor are you using obliterate (as you should be using death strike). Unholy gives you your choice of reduced taunt cooldown (Deathgrip), free +hit% for your spells (so deathcoil won't have a chance to miss) and morbidity (boosts the damage and thus, threat, of your runic power dump: deathcoil). Taking frost as far as Lichborne, however, means you can activate Lichborne, and then cast deathcoil on yourself to get a free heal if you're low -- a neat trick, and handy if you're low on health after a big boss burst.
Glyphs
Prime Glyphs:I would go with glyphs of Death Strike, Heart Strike, and Deathcoil for your prime glyphs, though depending on playstyle you may prefer to swap the latter for Runestrike (though I'll always take +damage over +crit chance, since the former is more consistent) or Death and Decay if you want additional AoE threat.
Major Glyphs: Dancing Rune Weapon is a no brainer, but beyond that your choices highly depend on your playstyle.
Minor Glyphs: I'd Glyph Blood Tap and Death Grip, but these are minor glyphs, and won't have much effect at all in regards to your tanking.
For further discussion, I'd point you toward the Elitist Jerks' Death Knight Forums, specifically the tanking thread, which can be found here.
Best Answer
In my opinion, two-handed works best in pvp. Make sure you use the right glyphs, for example 'Anti-Magic Shell' and useful talents include 'On A Pale Horse' and 'Improved Unholy Presence'.
Personally, I've done better with a Frost spec in PvP but provided you use the right spell rotation, you should do fine with Unholy.
Hope that helps.