What is a "flex tube" on a 2004 Volkswagen

My friend has a 2004 Volkswagen. She began hearing a noise coming from underneath the car. She took it to a mechanic that is listed in the yellow pages and he told her that the flex tube is damaged. He told her that Volkswagen sells the entire unit for around $1,000.00 but that he can cut the damaged flex tube out and weld a new flex tube in for around $125.00. She asked me to ascertain what a flex tube is, if $125.00 is a good price, and if this is a good way to have it repaired. Can someone advise me on this? Thank you.