Yes. Since it contains no personal data (like CD keys or account info), sharing the stock defaultcfg.cfg is perfectly safe. Your friend can use it to reset their game too.
Instead of editing default.cfg , create an autoexec.cfg in the same directory. This file executes after default.cfg but before your config.cfg , allowing you to override defaults safely. For example: defaultcfg call of duty black ops 2
C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Black Ops II\zone\english\defaultcfg.cfg defaultcfg call of duty black ops 2