What should you do if you accidentally give a friend severe poison ivy?

I had to take pictures for a photography project, and I asked my 3 friends to come to an Arboretum so I could take some pictures. For one of the pictures, my and 2 of the girls thought it would be cute to have the 3rd, Lacey, walking down a path that went off of the trail. It was still a dirt path, just not as wide and plants got closer to the edge. We joked that she was going to get Poison Ivy, but once we got the shot we moved on, and for the next 2 weeks everything was fine (poison ivy wise) with all of the girls. I got a call from one of the girls (not Lacey) saying that she had severe poison ivy that was highly infectious and had to go on steroids. This girl pointed out that it must have been from our trip to the Arboretum, and I don't know any way she could have gotten it either. I feel so terrible that her family has to pay so much and that she's suffering. We both do dance and she can't dance for the next couple of days. I feel terrible and don't know how to make it up to her. What should I do?