If you continued drawing unaware that the UNITS change had been made, AND you continued entering your lengths and dimensions as if they still were in millimeters, the change probably had no effect on your work.  Just be sure that you correct the UNITS settings before going much further.  You might also want to ask that other person what they were thinking when they made the change.

A random check throughout the drawing of dimensions is probably a good idea too.

Dave.

Without knowing how your two drawings were set up it's kind of hard to predict what's going to happen.  But I would suggest you create some instructional example files for yourself as a teaching aid.  They may help you understand this more quickly than anyone here could explain it.

Start a new drawing using the ACADISO template.  This will be decimal lengths and millimeter insertion units.  Create a square box 25.4mm x 25.4mm and dimension it.  Save the file as SQ-MM.

Now start another drawing using the ACAD template.  With the UNITS command set the length type to Decimal and the Insertion scale to Feet.  Import or attach your SQ-MM drawing.  Zoom extents if you can't see it.  Use the MEASUREGEOM command to check the lengths.  They should be 0.0833' (1" or 25.4mm) long each.  AutoCAD did the conversion for you.

Start one more drawing using the ACAD template.  This time set the length type to Architectural and the Insertion scale to Inches.  Import or attach your SQ-MM drawing again and check the lengths.  This time they should be equal to 1".

I think this little exercise will go a long way toward helping you understand how the Units and Insertion scale settings work between drawings.

Dave.