RE: Marketing Form Designer Issue - Can't Load Custom Fields
Hi Anthony,
Actually form editor is a field of marketing form entity, which name is Form definition, and it's controlled by a control called formEditor,
you can find the field properties in Customization > Entity > Marketing Form > Information form.
This editor, as well as other editors or designers such as marketing email designer or segment designer, they're controlled by Custom Control entity.
So please check wehther your team member has been assigned necessary privilege to the entity for her security role, the entity is in Customizations tab.

After I have removed the privilege for my test user, it also encountered same error as yours even if he could see and select marketing template.

Regards,
Clofly