That's a tough call. If you know the person fairly well, you should have some kind of idea how they will react if you approach them positively about their horses. You might want to conveniently stop by while you're riding your own horse if that's possible. It's really easy to start talking about horses when you have your own right beside you. Then, as Ricci posted, you could offer up some avenues of action without accusing them of anything.
It really does depend on how they take your "chat." You could even just ask how they are doing with their horses. Maybe you've noticed that they've been busy and commiserate with how YOU find it hard to balance everything in a day too. It's so easy to lose track of time and when the last time the hooves were done, worming was done, etc. etc. In a general conversational sort of way. You kind of have to feel it out. If it feels awkward or negative in any way, don't do it. Maybe there is a mutual aquaintance that you could mention it to and ask that person how you could help?