They were enraged that their female friend was getting lewd text messages from a stranger.
So a group of male students from Singapore Management University (SMU) decided to save their damsel in distress.
They set a trap for the man on Wednesday at SMU - and he walked right into it.
The New Paper was alerted to this by SMU student George Ng (right), 23, who's doing a double major in information systems management and finance.
The Year 2 student said he was "the planner" behind the ambush, which involved about seven other male students.
His female friend who was allegedly being harassed is a fellow Year 2 student in SMU's business school, he said.
Mr Ng said that over the past year or so, his friend had been "stalked" by a man.
She knew neither this man nor how he had got her number, her friends said.
"He was stalking her Facebook, her Twitter account and he was sending her weird, disgusting SMSes," said Mr Ng.
The New Paper was forwarded some of these SMSes, which had the man asking if the woman would have sex with him.
The woman's friends said that the man's SMSes became more frequent, and he also started calling her constantly.
Said one of the friends, who wanted to be known only as Mr Tan, 22: "She called back once and tried to ask the man questions, but he said nothing."
Unable to bear it any longer, the woman, who is in her 20s, finally told her friends about the man a few weeks ago.
That's when their protective instincts kicked in.