By adding needed module to python sys.path. Fixes this:
$ rdiff-backup-statistics backup-dir
Traceback (most recent call last):
File "/home/bfo/.nix-profile/bin/rdiff-backup-statistics", line 24, in
<module>
from rdiff_backup import connection, regress, rpath, Globals,
restore, \
ImportError: No module named rdiff_backup