Hi remy,
This is a tricky ethical area; the Schiff Hardin lecture series goes into some detail on it as well. Essentially the idea is that if the architect were to offer specific direction regarding site safety (which is the sole responsibility of the contractor), the architect in offering such instruction could informally and unintentionally establish a duty, or at least the appearance of it, of site safety. And even if this duty is even informally established, and a worker falls, conks his head, and sues everybody later, he could successfully sue the architect by saying "she told me to put my hard hat on, so she was assuming the duty of site safety, and since I had an accident regarding site safety, she is at least partly liable for damages from this accident."
The Schiff Hardin lawyers recommend, if you see a bad safety situation on site, to immediately tell the contractor, and let the contractor correct the situation. You could establish verbally "this is just me the human being talking here, and not me the Architect or me the employee of XYZ Architects, but aren't you concerned that that worker doesn't have his hard hat on??" So you can be a good human being and express concern to have the contractor correct the unsafe situation they are responsible for, and protect yourself from the liability of unintentionally taking on the duty of site safety.
Those lectures really are pretty handy for learning the real-world application of some really dry contract law. Sorry I can't recall which specifically addressed that issue, but if you look through the lecture notes you can probably figure it out. Plus if your firm allows headphones you can listen to the lectures at work.
http://www.schiffhardin.com/design-buil ... pring-2012