In this instance the Client wants to know if he use a Cat 6 Ethernet for a Profibus application. My first reaction was 'No', but seems like its been done before with no issues.
The short answer is no, if it is really Profibus. The longer answer is that for short runs with only a few drops you can use THHN conductors and it will work.
Profinet on the other hand, is just ethernet cabling.
Technically, you CAN do anything you like. But in my experience, when you "roll your own" networking cables, and something doesn't work right, the first thing they will blame is the cables.
And also in my experience, 90+% of the time they are right!