You should be very happy with the generous and unneccessary offer they made to you. The servo "defect" you described was because of a CRASH. Your wife shouldn't be attempting a roll on her fourth flight with a high wing trainer that doesnt roll well to begin with. I feel it is poor judgement on your instructors part to encourage this. A loop should be attempted before a roll.
I have always had first class service from horizon and them giving you a free plane proves this.