I had worked for a leading OH company for 4 years and all too often managers would refer cases based on less than 5 days absence in a 12 month rolling period, which was costly and unnecessary in my view.
I now work in HR and we don't actually do pre-employment checks of this kind. We will use OH if people give us cause for concern with repeated absences or have been off work with some time. We use our own judgement on a case by case basis.
Having myself previously suffered from a period of Depression following a family breakdown, and having family members who work in Cognitive Therapy / Mental Health capacities I know that 1 in 4 people at any time suffer from depression in one capacity or another. I also know that there is far too much stigma attached to it and having been able to perform well in my own role, it is possible still to perform in your role while recovering. Some people are not always able to stay in work but one period of absence is not necessarily cause for any major long term concern.
I would also add that I never had professional support / guidance from my family as often, people close to you are too emotionally involved to professionally support mental health sufferers.
For people who take repeated individual days off owing to this condition, then this could potentially be cause for significant concern.
You don't state what capacity you have received the referral in - are you an HR Practictioner yourself? If so, if you prefer you could refer to your OH provider (an independent provider if you don't have one) for a Pre-employment Health Check which will be able to give a stronger indication for "fit to work" purposes.
Common sense would say that if the individual has only had one period owing to this condition 3 years ago and has been able to work continually since, then it's a pretty safe bet that they will be "fit to work" so save the company money and use appropriate judgement.