> Download http://aminet.net/package/dev/mui/MCC_TheBar-26.21
> or https://www.morphos-storage.net/?id=1600933 package,
> it contains the latest Toolbar.mcc too.
> You probably need the above class(es) with any versions of YAM.

v26.21 of the archive contains latest Toolbar.mcc which is just a wrapper to TheBar.mcc. So indeed, if he installs this archive then old programs expecting Toolbar.mcc as well as newer programs directly using TheBar.mcc should work. Newer versions of YAM work without having Toolbar.mcc installed (no matter if wrapper or 'real' version). Older versions of YAM don't know TheBar.mcc, so the 'real' version of Toolbar.mcc (v15.12) works, but instead installing both the new wrapping version of Toolbar.mcc and TheBar.mcc should work as well of course.
Overall, installing all classes of the v26.21 archive is probably the easiest way as then only one archive is needed and both old and new programs should work :-)