I think many people overlook the obvious in this scenario. Everyone goes to the Inquisitor for their answer.
Ascension is a wreck. Stay away from it.
My second recommendation would be to look into survivable careers. Tech-Priests, Explorators, Arch-Militants, all of these have access to Skills, Talents, Traits, and equipment that would make them far more survivable. Heck, even the Lidless Stare of a Navigator would be useful against a Horde of cultists or Fire Warriors.
Do you want someone to play an Inquisitor, a servant of the Imperium that can decide "Nope. We're not doing that Mission. I have a better idea…" and thus derailing your campaign? They do that, you know? Or, do you want someone playing an Inquisitor, a servant of the Imperium that is hamstrung and does everything the Kill Team does and is an Inquisitor in only name? Either example neuters the very spirit of an Inquisitor's purpose.
On the other hand, an Explorator, with interests in archaeotech, xenotech, or perhaps a Geneticist with an unhealthy fascination for the Xenos biology/physiology, makes more sense. The Rogue Trader Core Rules and many of its other supplements clearly show the characterization of an Arch-Militant as female, and I can speak from a GM's first-hand experience…those characters are pretty durable.
You friend could play a Gland Warrior (also Rogue Trader, outlined in the Into The Storm supplement).
If you really want to go the Ascension direction, there are Confessors, Storm Troopers, Vindicare Assassins (which uncounted GMs have lamented as being possibly the most "broken" PC career available).
The single biggest hitch in all of these is Armour and Wounds. Normal humans are lucky to get or surpass a total of 25, considering there are many other choices for Skills and Talents. Someone who's spent thousands of XP buying Sound Constitution has a pretty one-dimensional character. Normal humans have access to Power Armour, but the civilian models are nowhere near as protective as their Astartes counterparts. Civilian Power Armour can have its power source upgraded by the Forge-Masters of the Deathwatch, that's no biggie. The Apothecaries can't do the same for a human's physiology.
With Dark Heresy and The Lathe Worlds supplement you could have a Tech-Priest with comparable protection, weapons, and ability. But you would be looking at a Dark Heresy character that should rightfully be moving into the Ascension level of their Career (13k+ XP), and that doesn't even start to address this question: Are you willing to sit down, at great length, with this person while they build a multi-thousand XP Character from the ground up?
You could go with my first recommendation. Gender serves no purpose in the course of a Deathwatch game. Let her Play a Space Marine that is Female. Female Space Marines Would be no more likely to use their "feminine wiles" during the use of Interaction Skills than a Male Space Marine would attempt flirting. The idea of romance, intimacy, sex, procreation, lust, and physical desire are anathema to the Adeptus Astartes. So what, this particular Space Marine has boobs? Have you seen female body builders? Gross. If one of them wanted to "sex me up" I'd run until my legs fell off. If you (and your other players) can't hack the FSM angle, that's on you, not your lady friend. A Space Marine is a Space Marine. No one would be any more awed in the presence of a FSM than they would in the presence of a MSM.
You have a lady friend who wants to play in your game. Be a man, wear a pink shirt, and let her play a FSM. It's a game, not the end of the world.