How can I take the backup of all iDash reports?
You can run a sql query to get all the reports to back them up.
The following sql below will get all the reports plus the ones that are deleted.
select * from idash_config where type like '%report%';
to cut those deletes out, you can do
select * from idash_config where type like '%report%' and deleted = 0
The DBA should have a tool like SQL Developer from Oracle or something similar, which can easily export the select results into some other format, including as insert statements or CSV files which is recommended.