Have either of you tried the "Search" feature? Obviously not or you wouldn't be posting here.
There are 86 other posts all with threads that list the solution to this problem.
Try this:
Delete ALL Mabinogi folders from your computer: there should be two of them; one in C:\Program Files\Nexon\ and one in your My Documents folder.
Reboot your machine, then restart the install launcher, CHANGE THE DEFAULT INSTALL PATH (mine is C:\Program files\Nexon1\)
and continue with the install.
Mabinogi should install properly then.
Life is but a dream for the fortunate, for the rest, it's a NIGHTMARE!