Generally speaking, changing employment conditions up to and including firing your friend does not discharge his obligations under a non-compete agreement. There is no constructive discharge of non-compete agreements. There should be some kind of a time limit and usually a specific geographic limit or industry limit, otherwise an overly broad non-compete would bar your friend from any employment ever, essentially making him completely dependent on his current employer. This has been generally found to be against public policy and such agreements are void, but properly restricted non-compete agreements are definitely enforceable.
As for whether an ex-employer would attempt to enforce a non-compete agreement, that likely depends on the people involved and situation. Do they have a history of doing so? If so, do they pursue only executives, or any sales person? Do they have certain competitors they will sue to block employment with? Most employers do not attempt to enforce non-compete agreements in most cases, but certain egregious actions such as taking customer lists or poaching ex-coworkers can goad them into doing so.
What is the goal here?