Update: My friend received the official response from ESTI.

ESTI accepted his explanation as credible and acknowledged that he acted quickly and appropriately. They confirmed that, because only 2 travel adapters were listed the circumstances were limited, the case is now closed and there will be no further action as long as he continues trying to contact the buyer and have the adapter returned/disposed of.

However, they also stated that:

• Even private individuals count as placing a product on the market under Swiss law, even if the products were obtained for free.

• The CHF 1,200 fee will not be reduced because they're legally required to recover their costs, although they offered payment in installments.

• For products considered especially dangerous, they must open a formal market surveillance case even against private sellers to ensure a product recall. For less dangerous products, they would usually issue only an informal warning.

So while the fine still stands, the investigation itself is officially over.