With an external hammer, you get the option of SA (single action... only drops the hammer) or DA (pulls the hammer back and drops the hammer).... with an internal hammer you only have DA and therefore always have a longer trigger pull.
Yes, with an internal hammer, you won't catch it on anything, but if she's carrying in a purse (I won't comment on whether that's a good/bad idea...it's been done and redone on other threads), then she should have a separate compartment that the firearm goes in so she's not hunting around for it when she most needs it. Therefore, the whole idea of it "catching" is a moot point. If she is pocket carrying, she should get a pocket holster and therefore won't catch it on anything either.
Personally, I like the option of SA. And since you can avoid the idea of catching it by carrying it properly, than I would go with the exposed hammer.