If she had no problems with the .40 and didn't find the recoil objectionable, I would say have her check out the reliable guns in 9x19mm through .40 until she finds one that fits her hand. (Of course, if your .40 is a steel frame service piece, before plunking down money on mid sized "Combat Tupperware" such as a Sig P250C, Glock 23, or XD, try and get her to shoot something like that first -- the recoil impulse is different, and what may not have been objectionable in some steel frame CZ75 variant might be more than she cares for in a polymer framed compact.)
But if she can handle a polymer framed .40 (which I'm guessing your pistol is), she should have ZERO problems handling anything that size or larger in 9mm Parabellum, .40, or .45ACP, if it fits her hand and she likes the control ergonomics and layout. The gun that "fits like a glove" and kicks no harder than something she likes is probably the very best choice for her, regardless of caliber, trigger system (although I'm a fan of DAO or Glock-style striker triggers for newbie defensive guns; not as sensitive as a 1911, yet you get the same exact pull every time), magazine capacity, etc.
While I'm a die hard .45ACP fan, realistically, ANY modern major US manufacturer JHP in a normal bullet weight for the caliber (for instance, avoid 147 grain 9x19mm like the plague, unless you're shooting it through a suppressed SMG -- that's what it was designed for <grin>), and a standard "service caliber" (9x19, .40 S&W, .45ACP, .357 Sig, or for revolvers, .38 Special or .357 Magnum) will do pretty much the same, regardless of it's caliber. Hit location is WAY more important than the almost statistically insignificant differences in caliber.
For a gun whose primary use is home protection, concealbility is not really an issue -- "shootability" is. If it turns out she likes Glocks or Springfield XDs, well, a second, smaller gun is certainly not a kidney-selling experience if and when she decides to CCW.