Maintaining a physically active lifestyle is important for diabetic and obese patients, but how to motivate them to exercise and engage them in the long run remains a challenging issue. We aim to motivate their activities using fitness trackers and prevent relapse using peer influence. We conducted a study with diabetic and obese patients who used an activity tracker for four months. The 16 patients participated either as an individual or with a buddy. During the first month, most participants reported that they were motivated to conduct moderate-intensity activities. In the following three months, their number of steps significantly dropped, but the decrease is higher in the Individual condition than the Peer condition. The frequency of activity tracking slightly increased for patients exercising with a buddy, but significantly decreased for those participating as individuals. Patients with a buddy reported that competition, working out together, and nudging each other motivated them to exercise and keep monitoring their physical activities.