There's a flixbus every 1-3 hrs directly from WAW. Otherwise, he should take a train to the main station and change to a train to Lublin. Time-wise, the bus should be faster, but there are more train options throughout the day.
Not sure how a 16-year-old would not be able to navigate this by him/herself, unless they grew up in a bubble. This is Poland, not China, most young people speak some level of English.