A spurned businessman smashed up his Barnoldswick love rival's prized Audi after the victim told him the defendant's two children had started calling him "dad" a court was told.
Saul Root (45) was seen walking away from ex-friend Mark Langtree's A4, which has a private number plate, with a large spanner in each hand.
Root had "lost it" and smashed all the windows of the vehicle, causing damage to the tune of more than £1,000, at the victim's home.
Burnley magistrates were told how Root's former long-time live-in partner was said to have started seeing Mr Langtree when she was still with the defendant.
The defendant's solicitor Mr Geoff Ireland told the court: "He suspected she was seeing another man and it eventually came to light she was seeing Mark Langtree. That upset the defendant significantly, and his relationship came to an end, but moreso because Mr Langtree had been a friend of his. His life fell apart."
Mr Ireland said Root had not seen his sons for some time and had had lots of messages, particularly from Mark Langtree, who "has stepped into his shoes," saying "The boys are now calling me dad."
The solicitor told the magistrates: "You can imagine how he felt about that. He thought 'The only way I can hurt him is to damage something he holds dear.'"
The defendant, of Crestbrook Road, Stoke-on-Trent, admitted damaging the car on August 31st.
He was fined £200 and must pay £85 costs, a £30 victim surcharge and £500 compensation.
Prosecutor Miss Parveen Akhtar said after the damage, the victim had tried to speak to the defendant and ask him why he had damaged the car. Mr Langtree picked up a brush from the garden.
She added: "He tried to leg sweep him again and again, but he failed."
Mr Ireland told the hearing Root needed to go to the courts and do something about seeing his sons. He accepted his actions had been "a moment of madness."
The solicitor added: "Mr Langtree went for him with a brush and he realised he had gone completely over the top and left."