Based on the rules of negotiation for Alternative dispute resolution how should I rule on this case in regard to the facts listed below as the mediator?

The facts general facts are below

Mark and Susan Roberts divorced last year. At the time of their divorce, they reached an amicable agreement on child custody and property division without using attorneys. Susan received custody of their only child, Dierdre, age nine. Mark received visitation rights of "8 days per month, including one full weekend and one additional weekend day per month." Mark pays $225 per month in child support. For the first six months following the divorce, Mark and Susan had no problems with Mark's visitation rights. During the past three months, however, the situation regarding visitation has deteriorated badly. Mark and Susan are now unable to reach agreements about visitation. Susan is upset because Mark failed to show up on assigned days, and Mark is upset because Susan was unwilling to rearrange a visitation weekend so that Dierdre could see her grandfather who was in town on an unscheduled business trip. Mark's schedule is hectic. He is a successful building contractor and has numerous jobs that demand a great deal of his time and energy. Susan feels she has been wronged by Mark. She thinks Mark has abused her flexibility. Susan feels that Dierdre needs to spend consistent quality time with her father but feels Mark has taken advantage of her to schedule, cancel, and reschedule visitation dates. She is also tired of dealing with Mark, who calls constantly about visitation. She wants to get on with her life as a music teacher. On three occasions, Susan was forced to cancel music lessons with private students when Mark failed to show up to pick up Dierdre. Susan also feels that Mark is not giving enough attention to Dierdre's school work when she is with him. Dierdre now has homework at school and Mark seems indifferent to overseeing her assignments. Susan blames some of Mark's behavior on his compulsive attachment to his job as well as his sister, Julie, a very successful stockbroker. Julie and Mark are very close, and Julie has urged Mark to end his relationship with Susan. Susan has overheard Julie say to Mark: "Susan can always take care of Dierdre don't hesitate to change your visitation dates because Susan is the type who is always there for her child." Susan and Julie do not speak. Dierdre appears to like Aunt Julie.