Did he not pay taxes on the interest? I can understand that not disclosing it might be illegal, but if he paid interest on the income, I don't understand why it would be a big deal. Tell me more.
Generally, having $3 million in a Swiss bank account and failing to disclose it is considered a sign that something nefarious would perhaps be going on. At the least, it would cause someone in the financial industry to be investigated by his or her employer and possibly the SEC. Most likely the person would get fired.
It is considered a serious ethical breach. At my last employer, a rumor was going around that ownership was looking for someone to fire for being late disclosing stock accounts, just to make an example of someone, because they knew the SEC likes to use things like that as an excuse to effectively shut a firm down. There is no way someone who allegedly worked all his life in the financial industry doesn’t know this and failed to disclose it inadvertently.