SINGAPORE - The grandson of the founder of OG department store, convicted of harassing his wife's then husband who had brought the charge against him, does not have to pay the legal costs of the prosecution.

Lawyer Luke Lee, who led the prosecution, spoke to The Straits Times outside the courtroom but declined to reveal the costs, saying it was a "five-figure sum".

Following the decision by District Judge Ng Cheng Thiam on Tuesday (Aug 13), Mr Lee's client, civil servant Desmond Tay Kwang Ju, who turns 40 this year, will now have to foot the bill himself.

Kelvin Liu Chin Chan, 44, was fined $3,500 on July 15 for harassing Mr Tay, who was married to Liu's current wife, identified only as Madam Lin, at the time of the incident in 2016.

Judge Ng had found Liu guilty of one charge under the Protection from Harassment Act brought by Mr Tay, who is attached to the Singapore Consulate-General in Shanghai.

In his testimony, Mr Tay said he learnt of an alleged affair between Liu and Madam Lin when Liu's then-wife contacted him in May 2016.

She alleged that Madam Lin was planning a trip to Hong Kong with Liu the following month.

Mr Tay said that after being told of the alleged affair, he hired a private investigator and found out that his wife and Liu had travelled to Hong Kong from June 7 to 10 that year. However, Madam Lin later denied that she was cheating on Mr Tay.

To clarify the issue, a chat group was created with all four parties included.

But in the group, which was named Confessions of Marco Polo, Liu sent belittling and humiliating messages, calling Mr Tay an "idiot" and a "wayang king".

Liu also asserted that Mr Tay was lying about the nature of Madam Lin's alleged affair.

Mr Tay said Liu also sent him two photos - a topless picture of Mr Tay taken by Madam Lin in private and a photo of a shirtless torso with the face obscured, believed to be Liu.

Mr Tay said: "I think he wanted to let me know that he had something private of me and make me feel vulnerable. It felt like he was trying to say he was superior to me in physique, which is why my wife is with him and not me."

It was heard in court that Mr Tay's wife filed for divorce in August 2016, but he filed a defence and counterclaim on the basis of adultery.

During the trial, Liu claimed Mr Tay had made several threats to him through third parties. He also produced WhatsApp conversations in court in which Mr Tay called him "a real joke" and said he was prepared to tell everyone about the case.

Liu belongs to one of Singapore's richest families. His grandfather was the late billionaire Tay Tee Peng, and their family had made the Forbes list of wealthy individuals.

For harassment, offenders can be jailed for up to a year and fined up to $10,000.