The girl's actions are outside the control of the players. This should be hard, yes. But when you reduce things to a single die roll, that's not hard, that's arbitrary. You're replacing actual difficulty with random chance. This should be a dialogue tree puzzle, something you have to think through and consider. The die rolls should open up paths to make things easier, but they should not determine success or failure in and of itself. Not on such a plot point.
I've been discussing this in other topics already, but my feelings are pretty much summed up by the quote.
It could be handled multiple ways. It could be
1) an initial dice roll, opening up more dialogue options on success
2) a branching dialogue where you select maybe 3-5 lines, and in the end you roll with a target that is determined on your dialogue choices. The target could be raised, lowered or stay the same, depending on your choices.
3) as it is today (see next point)
4) along with any of the above options you could get additional dialogue options, or additions to the skill check based on the what information you uncovered.
A very relevant point in this case could be to prevent the Tiefling refugees to go into conflict with the druids - this would require a slightly different narrative than we have in game today, but it would definitely be worth considering for someone in Kargha's position. 
