The NetApp "My Autosupport" Upgrade advisor gives the full steps for upgrade and revert. It looks simple enough (Download 7.3.5 from now, copy files and use the download command) but I would get them to call support just to confirm there is nothing nasty waiting to suprise's them. Especially if they have been running DoT 8 for a couple of days.
Excellent advice to contact NGS when performing an ONTAP reversion. The Data ONTAP Upgrade Guide has an entire chapter devoted to all the features impacted by a reversion...but contains no procedural steps to perform a reversion. A service partner recently ran into an unexpected issue with deduplicated volumes which require running the 'sis revert_to' command before running revert_to. This is not mentioned in the Upgrade Guide (I'm filing a doc BURT to fix that) and the only information on 'sis revert_to' is in the 8.0.1 Command Reference/MAN pages.
Reversions are generally rare, but the new FAS3200 shipping with ONTAP 8 have required a lot more reversions. The systems are now shipping with 7.3.5 as long as the Sales team requests that version.