Here goes...

I use a DK tank for my shaman team and also use her as a raid tank for my guild. Now I just looked up Mystorie on Mal'ganis and found a blood elf Death Knight, so I am assuming that is your wife's toon?

If it is I see she is Frost specced and does not have the talent for corpse explosion. If she has just changed, then good move, because I don't find it useful at all. I do find a frost spec the best for AoE tanking now since the unholy 3.2 nerf however.

With a frost spec, the best AoE rotation is: DnD-IT-PS-Pest then I tab to another target and HB-Pest-BloodBoil. Always tab to another target after blood boil. This means pest is always refreshing your diseases without having to reapply and also you are spreading threat from your single target attacks evenly through the group.

In frost spec I dump runepower into runestrike and frost strike. Because rune strike is cast on next swing you can make macros for every ability or at the start of a castsequence with /cast runestrike. If it is available, it will be queued up without altering your rotation. She should use runestrike at every available opportunity because as well as being her highest damage attack generally it has 150% threat.

I do find howling blast is brilliant for snap AoE agro however, as long as you wait for all diseases to be on targets, you should be fine to go balls out AoE dps without fear of pulling agro.

For single target rotations, I use HB-BS-BS-OB OB-OB-OB and RP dump is the same as above. This rotation is assuming howling Blast is glyphed to apply diseases. Alternatively substitute HB for IT at the start of the rotation. Note you don't need to use plague strike in a single target frost rotation, the unholy rune is better spent on the obliterates.

Finally, glyphs. Your wife should have Howling blast, rune strike and obiterate glyphed or swap howling blast out for the rune strike glyph. There are also some significant gear issues to look at, but obviously that will come with time

Hope this helps.