At this point then, I would ask her how she feels about raiding in general. Like 2 months ago we did just that with a guy who just wasn't improving his DPS despite like 10 ilvls of upgrades, kept dying early, etc. Eventually it came down to "Look man -- Are you having fun? Do you even want to be here? What is the deal?" And the answer was "No, I don't really care anymore." So we replaced him.
Or see if she's tired of Warlock and maybe has another class she really wants to play instead. For 5.2 I was getting kinda bored of my Rogue, so I switched to my Monk and ended up actually taking over MT duties from my RL friend who was tired of tanking and was ready to just quit WoW. So I'm getting a change of pace tanking on the Monk (Brewmaster is a lot of fun) and he gets to DPS now and get a breather.