Ms Ridley also spoke to Ms Cotter's sister Tracey-Lee, and followed her advice to induce vomiting.
However, by the time Mr Ridley arrived at her house with police, his sister was unresponsive on the bathroom floor. Ambulance staff were unable to resuscitate her.
None of Ms Ridley's family believed the overdose was deliberate, saying she had acted on her emotions following too much alcohol at the party.
Ms Ridley had a history of alcohol abuse, but had been addressing that in the two years prior to her death.
In his ruling, Mr Evans said there was not enough evidence before him to provide a clear view of her state of mind.
"The telephone calls made by her for help in the early hours of December 28, which went unanswered, underline the essential ambiguity of the evidence as to the knowledge, understanding and intention on her part in taking [the painkiller] and point towards a 'cry for help'," he said.
There was not enough evidence to give a firm ruling on her death, he said, giving an open verdict.
However, he called for a review of the telephone helpline services in New Zealand, recommending the establishment of a 24/7 helpline.
The inquest heard evidence of the 108 different helplines offered across the country.
Murray Edridge, deputy chief executive of the Ministry of Social Development, today said a review of the multiple helpline services was planned for this year "so they continue to provide the right level of help and support for people in need".
"The terms of reference for the review are still being finalised."
The Ministry of Health said a national helpline service, Telehealth, is due to start operating from July 1, 2015.
It would "improve public access to a range of unplanned care, advisory, counselling and referral services through a multi-channel approach including telephone triage and telephone advice, text, email, phone applications, social media and web-based services", the Ministry said.
"The National Telehealth Service will be available 24 hours per day, seven days per week by telephone, text messaging or through on-line tools."
It did not respond to questions about whether Ms Ridley's case had been a factor in the development of a single national helpline.