I agree with @WesleyLong that this has the appearance of the company trying to "pull a fast one." It looks like they are trying to layoff the OP without actually performing a layoff.
Assumption: There is no employment contract or union contract which has rules about this kind of thing.
First - All communication should be in writing, or be acknowledged and confirmed in writing. The instructions from the boss should be confirmed with an email, with a copy to Human Resources and to his supervisor. The email should ask for a projected date when work will resume. If the employment includes benefits, the email should inquire about the status of those benefits.
Second - The friend should check on the unemployment insurance rules in their jurisdiction and apply if eligible. Terms to look for are "reduced hours", "furlough" and "lay off." The fact that the company doesn't call it a layoff doesn't mean that it isn't.
Third - The friend should immediately begin looking for alternative employment. If this company remains attractive to the friend, then perhaps looking for temporary employment is a reasonable alternative.
Fourth - Your friend is under no obligation to remain available or to inform the company of their employment situation, and I would do neither. If called to return, your friend can make the decision about whether to do so or to decline.