I see two questions but I will give a three part answer:
- You do not need to access your ex-colleagues computer. If you had it and logged in on that computer (using your own credentials) then it would not change anything and you would still get the same permission denied message.
- Yes, your network admin can delete it for you. If this is a once off situation then that would be the quickest and easiest solution.
- Look into why this folder was only owned by a specific person. One would expect network folders to be owned by groups. E.g. all sales people would be added to a group (lets call it 'sales_dir_access', but the name does not really matter) . Reading, writing and deleting folders in the sales directory would be allow by all people in that group. This is likely already the intention but the folder somehow slipped though.