If your tensioner was bad and stuck in position it could let the belt slip. I had it happen on my mitsu but thats a completely different design. The belts will stretch but the tensioner should keep it tight. '99s have non interferance engines so if it slips or breaks there should be no damage. Should not have to take off the valve covers to line everything back up again unless the belt breaks or slips ALOT. I wouldnt think it would slip more then a couple "teeth" for the cams though. With everything else lined up he should have been able to see it was off a bit and line them back up just by the timing marks.