Tell them to go outside with a water hose with some good pressure behind it and spray it directly around all the suspected leak areas. If its leaking it doesn't take much time for the moisture to start showing up on the inside.

If still not satisfied, ask them to call their local fire dept. and ask them if they would be so kind to bring out the firetruck and hose it down. It sure they can crank up enough pressure for the leakage to show up. (If they don't take out the windows and doors completely)

Rick