What is the ruling on my friend's work in the Civil Defense, which requires shaving the beard as an internal regulation? And upon whom does the sin for that fall? Was the Messenger of Allah (peace be upon him) given a choice or was he compelled, and could he have disobeyed Allah?

Shaving the beard is unlawful according to Islamic law, except in cases of necessity. For instance, if abstaining from a job that requires shaving it would lead to the destruction or near-destruction of a person or their dependents, then in such a case, there is no sin upon the one compelled. As for those who command or compel others to commit sins, they incur sin because of that.