I wouldn't be too keen on this as an employer - I"m not particularily sure `i'd agree to it when I was a TU rep either.

I'd be saying to both parties that you would like to discuss any medical issues with the employee for two reasons that jump into my head.

1. Do they really want to discuss confidential matters with their rep and what would be the advantage to doing so?
2. As an employer it means the rep remembering what was said, and in any case only YOU the employer can make reasonable adjustments if necessary or refer them to an OHP service and so on. A good opportunity? for Chinese whispers.

What I would be happy to have happen is having the rep there with the employee when YOU meet with the rep.

I'd say it depends on what's being discussed. If it's simply the absence - how long it has been and what steps you propose to take to address it - then I'd say you should be fine to discuss it with the union, but be sure to follow up all conversations in writing to avoid any misunderstandings (which is always true). I found, when I was a regional officer, that members found it easier to hear the procedures from me - who was literally paid to be on their side - than from the employer, even if we said the same thing. There was often a residual assumption that the employer was lying to their advantage or trying to deliberately confuse their employee.

I hasten to add that this was almost never true (with one awful exception).

Where I would draw the line, as David says, is discussing any medical aspect of the absence. I wouldn't be discussing their condition, welfare needs or reasonable adjustments without the employee being the primary point of contact.

In reply to David Perry:

Thank you so much for you reply, this is really reassuring.

In reply to Robey:

Thank you so much for you reply too, you've both been a great help, very reassuring.