The first thing to check is if yours Apple Watch battery health is below average. On your Apple Watch, go to Settings> Battery> Battery status and see if it is below 80%. If so, you should consider an Apple battery service. On the same page, be sure to check if Optimized battery charge is enabled. It is enabled by default, so there is nothing to worry about for most people. However, if disabled, Consider reducing battery aging.
Another thing to check is if the watch battery suddenly collapses. On the clock, go to Settings> Battery and check the chart to see if there are any sudden drops you can try correlate with which he used the device because at that point, so you can determine if there are any something you need to be doing differently.