ともあろう

ともあろう follows a noun for a respected role – doctor, judge, a major company – and criticises someone in that role for doing something beneath it: "a doctor, of all people". It is followed by ひと人 or もの者 and が, and the sentence ends in disbelief or reproach.

The second half usually ends in とは, なんて, しんじられない信じられない or べきではない. ともあろう only criticises; it is never praise. The noun has to carry real standing, so an ordinary noun sounds wrong: ×学生ともあろう人が. For an organisation, write もの in kana.

ともあろう or たるもの

Both name a role and the standard that comes with it. たるもの states the standard as a rule. ともあろう complains that someone has fallen short of it.

ともあろう is formal and written – news commentary, editorials, a stern rebuke. In conversation the same complaint is きごう〜なのに: せんせい先生なのに、じゅぎょうちゅう授業中にねる寝るなんて.