It really depends on how proficient she is in MMO games, or rather how good she is with reacting appropriately to threats and knowing her class. Basically, is she any good at games?
If she is then I'd argue another healer would probably be perfect, especially if you both pick a healer that's good at kiting (Shaman/AM, Zealot/RP). One of you can spec more damage/utility/hybrid with the other being a pure healer and you should do very well.
If you think you'll be having to do most of the heavy lifting then you
will need a guard, 50% damage split is too good at keeping people alive. An offensive tank would be the best in that case, I'd argue an IB/BO/SM would probably fit best. You can throw a guard on her, kill whoever is bothering her or knockdown/knockback any additional threats/stuff you can't kill.
I guess a RDPS route would work well if you pick something that's decently self-reliant, has a heal debuff and can kite while still maintaining pressure. So a SW would be perfect (they have a ranged knockdown, a melee disarm, a ranged silence, an AoE snare and a morale knockback), alternatively a SH with a few less tools but more kiting potential.
Pretty much any career will work with 2 people playing together well, it's a question of how well can you do/what impossible odds can you beat. Some combinations are better at facing impossible odds than others, though they might not be as fun to play or you might not enjoy the careers as much.
But again, the value of Guard is immense, if you use it well any groups of comparable/slightly higher number attacking you will be automatically defeated provided they aren't using guard/aren't grouped.