In the C++ row group metadata object, the offset of the row group in the file is stored, but not exposed to users. RowGroupMetaDataImpl has a field file_offset and a method file_offset() that exposes it. But RowGroupMetaData does not have a file_offset() method. This seems odd, most other fields in RowGroupMetaDataImpl are exposed by RowGroupMetaData.
This issue is similar to ARROW-3590, but that issue seems pretty stale and is requesting a python feature. I think this issue is more focused and detailed.