timing belt on a seat ibiza

[silvermk4polo]

Sorry for posting this but a friend is in need..... can any one help, ok this friend of mine s belt tensiner snapped a coulple of days ago, so naturally we changed it along with the water pump, howerver wen the new belt was fitted it now jumps off around 10mm, so now were faced with a problem several machanics cannot find out why it doing this. the belt has allready worn with only 2 mins max of running. ive only posted this as i know mine and his engines are the same, please help as its driving him crazy. cheers for any info ash