There is no way to directly back up Forms due to the lack of established support for Microsoft Forms in the Graph API that we use.
A possible workaround:
1) The customer can set up a Logic app which saves Form responses as a file to their SharePoint document library or as a Microsoft list.
2) The files with Form responses are backed up from their SharePoint, as part of the SharePoint backup. Just make sure that the destination SharePoint site is included in backup (in case only certain sites are selected).