Depends who it is. There was a man at my yard who didn't know anything about Horses, he got one because he liked Western films and his ex had one. He brought a 16.2 WB who was a real sweetie. I was forever giving him advice without being asked as he didn't know any better, fair do's to the bloke though, he always listened and asked me why his way was wrong. I tried not to be all knowing, but unless he was told he would never learn. He was quite a nice man, he has now left the yard and moved his Horse onto grass livery, I just hope he remembers all our advice!
For example, one day the Horse was kicked in the field, nothing serious but did have some swelling over shoulder, vet told him to cold hose every day for 15-20 minutes. On the second day of hoseing, I turn up to see the Horse tied at the Hoseing area, the man had tied the Hose to the Horses leg using twine and then wrapped more twine around to hold it up. The twine was so tight I couldn't fit my finger inbetween the leg and twine. I cut the hose free and went off to find the owner who was in his stable. Conversation went something along the lines of this:
Me: "Hiya, the swelling on your Horses leg looks much better today, but please be careful about tieing a hose to his leg
Him: "Yea it does, Oh his alright, he doesn't seem bothered, I just thought it would be better then standing there and holding it, when I could be getting on with jobs, why is it dangerous?"
Me: "yes it could be dangerous, he looks relaxed bless him, but if he were to spook and pull away the hose would be stuck to him, which will make him spook even more, he could also hurt his leg from the twine as theres so much on there I doubt it would snap"
Him: "Oh I didn't realise, ok then" *off he goes to stand with the hose.
If I see something potentially dangerous then I will certainly jump in (A new livery was leading their horse in with the rope wrapped around their hand as an example)
When anyone is having a problem then I will offer a way to solve that problem. Up to the person if they want to take the advice or not. Generally they do.
I don't say anything unless asked and then I try to be diplomatic, unless a horse is suffering in which case id expect YO to take the action necessary. If I see someone struggling ill just smile and say alright... Then if they want to say something then. The worse thing is coming across like a know all.