One of the easier ways to deal with a shammy who is both a DPS and a Healer is dual-spec.
One day of dailies will easily pay for it, with a team of five.

In Elemental spec, the resto has the exact same hotkeys as the rest of the team.
I have this shammy drop the mana totem as their water totem.
In Resto spec, the water totem is either mana spring or mana tide.

In Elemental, like the other shammies I have a key for each elemental to LHW, assisting the tank's target.
In this way, I can have the Elementals either spot heal here or there, or assist the Resto with some heals.

When the Resto shammy is in Resto spec, I have assist heals and targeted heals.
I have five macro's, keybound to Shift F8 through Shift F12.
The macro's are written with the condition [nomod:alt] Riptide; Healing Wave.
Basically G1-G5 are just Shift F8 through Shift F12, which is Riptide on a specific target.
And G7-G11 are Alt + Shift F8 through Alt + Shift F12, which is Healing Wave instead.

The Resto has Chain Heal on the same key as the Elementals.

So I have the option of Lesser Healing Wave (all shammies, any spec).
Chain Heal, all shammies (either spec).
Riptide/Healing Wave (with Resto in resto spec).




If you wanted the healer to DPS, you could write macro's using the modifier:ctrl or mod:alt arguement.
That way, they'd help with dps, if the dps key was spammed with the correct mod.
But otherwise not help, which leaves them free to heal unhindered.