I suspect your friend is down-shifting too early, resulting in excessive engine rpm's when he lets the clutch out to engauge fourth gear. Oil leaks out of the valve cover when rpm's are above 4000 in the 4L, I assume the same sort of thing happens with the 2.5 L engine.

94 YJ, SOA, 2-1/2 Alcans, ARB-front, Detroit-Rear, 4.56:1 gears, Oasis Trailhead compressor, 4:1 Terra Lo, 37x12.50x15 SSR's, 8000 lb Ramsey, & etc.