If they have an Ultra 1, it could be nearing three years old. I've had a S2, S4, S6 and U1 then U2. My batteries on the 6 and U1 in particular began to falter after about a year and a half (I found a discount deal on a U2 with SpO2 and sprung for it because the Ultra was really crumpling, lost 12% in just over a year). There are several reasons why a Watch battery could degrade, but I'll name two—heat (it's generally the device most exposed to the sun) and fast drain/recharge (people exercise with it and the active tracking drains the battery quickly, then they place it on the charger and it recharges quickly). The Watch's battery life is already not-wonderful out of the box, so any degradation seems so much more pronounced than the iPhone, iPad or Mac.