Assuming that she does indeed have dual specs, she should absolutely be trying to heal in the Resto spec over the Enhancement spec. Typical fashion is a 1 hander and shield as Resto (or 1 hander and caster offhand, if your luck with caster shield drops is poor).
Back a few patches ago, there were a bunch of forum threads spread across the multitudes of WoW forums that were raving about a new "Resto spec" that picked up Dual Wield from the Enhancement tree in favor of going deeper into Resto. Allowing the use of 2 weapons significantly boosted your spell power, as you could use a 2nd weapon with 400-500 more spellpower than any shield at the time had to offer.
Blizzard quickly nixed that by making all 1 handed caster weapons "Main Hand" only, which means there is no potential for putting 2 caster weapons in both slots.
Additionally, in the upcoming Cataclysm expansion, Restoration and Elemental shamans will be completely unable to dual wield, regardless of whether it's better mathematically or not. Enhancement shamans will receive the ability to dual wield at 10 automatically: it's no longer a talent. The other two specs will not receive this ability.
Lastly, 2 handed enhancement has always been more of a leveling/pvp niche than anything. In vanilla WoW, Enhancement shamans didn't have the ability to dual wield at all, so they were forced to use 2 handers. And of course, in the current game, you don't receive dual wield until you can pick up the talent, so you're stuck with 2 handers until then.
There really shouldn't be a time where you're using a 1 hander and a shield as Enhancement except in PvP when you need to switch for defensive purposes. Your DPS is drastically reduced, and while you can take more hits than some other classes due to having a shield + mail armor, you don't have the threat output or survivability/abilities of a traditional tank class to make it work in the long run.
Edit: Forgot, since you kinda had an additional query contained within the initial question in regards to the Blocking/Parrying thing.
Without getting into too much detail, your dodge/parry/block is listed under the "Defense" tab on your character stats screen.
As an enhancement shaman:
- Dodge is affected by agility and +dodge rating.
- Parry is affected by +parry rating (also you must have the talent that enables it).
- Block CHANCE is affected by +block rating.
- Block AMOUNT is affected by +block value and strength.
- +Defense rating increases everything listed above except Block Value.
Enemy parry rating is important to note too. Most mobs in the game have the "parry haste" mechanic turned on (only some higher end bosses and casters do not). When you attack a mob, and it parries, the next attack it makes will be much sooner than it's default swing timer. This applies to player characters as well. As a dual wield class, hitting the enemy more per fight tends to result in seeing more parries, which means they have the potential to hit you more. I wont go into detail about expertise and hitting behind/to the side because there's way too many numbers to crunch in this small space.
I might suggest the forums at elitistjerks if you're looking for the exact details of all of those mechanics.