I'm at home now and don't have SW here to test this, but I'll try to help. Go to the assembly and edit Part B. Pull the roll-back bar up above your plane. Create a 3d sketch with two lines on the face of Part C, and close the sketch. Exit out of the assembly and open Part B. Open the 3d sketch, delete all external references, and use the Smart Dimension tool to dimension these lines from existing geometry until they're fully defined. (click on entities and when the dimension box pops up click OK to whatever dimension is there, being careful not to move the lines from their original position).
After exiting the 3d sketch, with the rollback bar right below it, create a new plane coincident with these two lines. Move all sketches and references from the old plane to the new one and delete the original plane (maybe suppress it first until you're sure that all references to it are gone).
There may be a simpler way, but it's the best I can think of without SW in front of me.
Thanks for help. I tried your approach and came up with a slight improvement on the basis of yours:
1. "create a new plane coincident with these two lines." --------> create a 3D sketch plane and make it coincident with the intended surface in C.
2. Select the 3D plane and delete all the relations. Then make it fixed. Up to this point the 3d sketch plane is indepedent and actually coincident with the face in part C.
3. We can then insert reference geometry (plane) in B referencing the 3d sketch plane directly.
The only essential difference is replacing "two lines" with a 3d plane at first, all other steps are basically the same as yours.
I'm glad you got it to work. For some reason I never use sketch planes so that solution didn't occur to me. It does sound simpler than my suggestion.
Glenn's solution allows you to relatively easily change the location/orientation of the plane if you need to later.
Personally, I think the best way to handle this type of situation is to plan for it up front by making a skeleton or layout part in your assembly that has the required geometry and planes in place to allow you to define your "in-context" parts. In the real world, I often don't understand until too late just what I need.