Good morning and thank you for entrusting me to answer your question. I will do everything I can to assist you.
Proving age discrimination is never easy, especially in light of the U.S. Supreme Court's recent decision to raise the evidentiary threshold for such claims by ruling that a plaintiff alleging age discrimination must prove that "but for" their age, the same adverse employment action would not have been taken. (See Gross v. FBL Financial here: http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&navby=case&vol=000&invol=08-441
Basically, an age discrimination claimant can no longer prove that age was one motivation of several--they must prove that their age was the driving force behind termination above all other things, and they must prove this by a preponderance of the evidence.
The above noted, "not easy" does not mean impossible, and the facts you describe create a strong circumstantial argument for discrimination, especially because this is happening on a large scale and the managers being forced out have been high performers.
Your friend would be wise to consult with a local employment law attorney to determine whether his facts are strong enough to bring a legal claim and forgo his offer of severance. It may very well be that he has a case worth aggresively pursuing, but of course further inquiry is in order.
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