If there is a UV filter optionally attached in front of the lens- you unscrew that and get a new one- cost $10. If the original owner did not get a UV filter and the scratch is actually on the lens itself it could be quite expensive to repair though it may be worth going to a service center first to confirm that. There’s so many used DVX100s out there nowadays and in good condition- if you have the stomach to pursue a refund it might be simpler.
-Noah