She alerted a Rotorua Lakes Council City Guardian who, with members of the public, went to the aid of one of the trio, who had collapsed.
Wilson said it was his understanding the City Guardian performed CPR on the man before a member of the public, who was a former ambulance officer, took over until St John arrived.
"I went down when I heard the job come in and from what I saw the CPR was being done really well. They knew what they were doing and they kept him alive."
Council operations group manager Henry Weston confirmed a City Safe Guardian started CPR when the man became unresponsive.
"All our City Safe Guardians are trained in first aid and this shows the very human side of their role in keeping our community safe.
He said it was the first time the staff member had had to perform CPR.
"As an employer we're proud of their involvement in saving a life."
Benji Mafile'o was parked near the library waiting for his partner to finish work when he saw the drama unfold.
He told the Rotorua Daily Post he saw the trio smoke what he initially thought was a roll-your-own cigarette and 10 seconds later one of the two men appeared unconscious and "dead looking" on the ground.
He said the unconscious man was purple while a woman who was with them also appeared to be under the influence of something.
"She couldn't even stand. She was rolling to try and get her way back up. It looked like she was off her face."
Mafile'o said the man was lucky the incident happened during daylight and people could help him.
The coroner has two deaths on file from Rotorua last year believed to be linked to synthetic drugs. Inquests are yet to be scheduled.
A rise in police callouts linked to synthetic drugs sparked Rotorua police to speak out in December to warn locals were dicing with death if they continued to use the dangerous drugs.
How you can help
* Provide information to police about synthetic drug users, suppliers and manufacturers via Crimestoppers anonymously on 0800 555 111.
* If anyone is using synthetic drugs, or they know someone who is, seek help by contacting their local GP or call the Alcohol and Drug Helpline on 0800 787 797.
* You can also text 8681 seven days a week to speak to a trained counsellor.