.40 or .45
Well I am no expert on the .40 or .45 but I thought I would add my $.02. I recently purchased a XD45 compact, this is my first .45. I would say my XD45 is more pleasant to shoot than my Mill Pro 9mm. I was very surprised at how comfortable it is to shoot, I have shot 185 gr. Gold Dots and 230 gr. Blazer Brass through it. I have never shot the XD .40 but have witnessed others struggle with the recoil, no doubt standing behind someone shooting one and it is a much higher pressure round. I would suggest if she really wants a .40 go with something in steel and some weight behind it it makes a huge difference, or at worse the 5" model. I have small hands and the .40 is more comfortable that the .45 but I most definitely can't complain when shooting it, it feels great. The .45 has an extra round and if you desire the compact makes a great carry weapon. I hope this helps.
Good Luck and Be Safe!