Eloquent way to ask friend to pay you back?

today my dad was going to take my, brother, sister and i to a waterpark 2 hrs away. but it rained so we didnt go. so instead were going tomorrow, but my sister cant go because she has work. instead of my sister, i wanted to bring a friend because my moms not really a waterpark person and admitted she wouldnt go on many of the rides with me. so i asked if i could bring a friend along instead. my dad said it was fine but asked that my friend pay us back for the ticket that she would use and that we had already purchased in advance. i feel really awkward and weird about asking my friend to pay us back for a ticket. expecially since when we go out her parents buy both of us stuff and doesnt accept me paying them back. how should i go about asking her to pay us back? should i call? or would it sound better in a text? HELP PLEASE!!!!