My suggestion would be to start with a fresh Scene, fresh Canvas, and add your SkeletonGraphic to it, and test things out with that - this way you learn the basics of how to set the attachment/skin without any of your coworkers configuration/code adding to the complexity.
I haven't used SkeletonGraphic before, but I'm under the assumption that the code for setting skins/attachments would be the same (as it is for SkeletonAnimation).
Open the character you want to test it out with in Spine -> are you guys using Skins to configure how the character looks, or are you purely using attachments? If you don't know the difference, then you'll want to take a step back and do a bit of research on that.
If you're using Skins, here is the documentation for setting them through code: spine-unity Runtime Documentation: Setting Skins
And just above that section is the documentation for setting Attachments: spine-unity Runtime Documentation: Setting Attachments
Here is a link to a script I made that lets you set/add skins, which can be triggered through code and/or can be activated in Editor mode. It is set up for SkeletonAnimation but I assume you can just swap that for SkeletonGraphic. Note that this script requires Odin Inspector for some attributes (see note #2 in the post): Editor Multiple Skins Question
Ooo, had forgotten that another user had adapted my script and set it up for SkeletonGraphic - looks like they added a couple additional features too. Maybe check this out first: [suggestion] make default skin support combination.
