Need some advice

My parents are celebrating their 50th anniversary at a local church. A wife of a family friend does photography and has a legit business. We wanted her to take some family photos at the church and she refuses to charge us, we asked if we could tip and she gave in and finally accepted.

She does photos on the side, the venue is 30-45 minutes commute, she has contacted the church a few times to ask a few questions. Total time on site will be 1.5 hours.

How much is an appropriate tip? We appreciate her time but don’t want to offend her. TIA