A husband and... a child from the past

Jorika is a renowned lawyer in Blatná. She has also been married for more than thirty years to Tibor Jonáš, a busy and respected gynaecologist and head physician at the local hospital. One day, her husband calls to say he will be late from work and asks her to find his passport in his desk drawer to check its expiry date. Jorika has never looked through her husband's documents, having always trusted him implicitly. However, she now comes across a carelessly opened envelope containing a letter in which a certain Jan informs her husband that he is his son.
Jorika reads the letter again and again. According to the date, Tibor received it ten years ago. But when Jan was born, Tibor had already been married to her for several years…