Just taken some kitchen curtains to the dry cleaners from a rental prop.
They were so greasy they were standing up on their own
No amount of vacuuming would have cleaned them.
Your customer that makes curtains obviously has no idea that most people these days live like Pinky and Perky.
If curtains are left for years which is normally the case,they cannot be cleaned by vacuuming alone,you may remove some of the dust but they will still be dirty.
John