I'd level, either Boomkin or Feral - Cat, whichever is more fun.
You can instance, as a DPS with either spec, and do fine.

Down the road, I'd probably either main spec Resto, with Balance off-spec.
Or main spec Feral - Bear, possibly with Feral - Cat off-spec.

As a tank or a healer, you'll have much shorter queue's then as a DPS.
And you have the DPS spec as an option for PvP, quests, etc.
But then again, it depends if she will be inserted into a team at all, and what role the team needs.