It is always such an awkward situation as you don't want to just totally offend them and make the whole thing worse! I think I will just keep having conversations with her, keep quietly asking questions and trying to get her to think about things...
That's what I was going to suggest as well, because this is the way I would want someone else to deal with me if he was concerned about my horse keeping. And I think that at times people might be concerned, for example when Titum refuses to eat hay in spring as soon as the grass starts growing, and then he gets very skinny. He has access to hay 24/7, but if he does not eat it, there is not so much you can do. Probably the situation is a bit different in the case of the woman at your place, but what I just wanted to say is that sometimes there are reasons that are not immediately visible for someone who is looking from outside, and then it does not feel so nice to get accused of neglecting your horse when actually you are doing all you can, or at least everything you think is normal. I guess she does consider the situation to be quite normal, so indeed, trying to understand her view and then perhaps help her modify it seems like a good strategy.
Anyway, just wanted to say that I think it's wonderful that you are so helpful and at the same time not intrusive. In other forums I have see the most awkward scenarios, with one person describing such a situation and then several other people jumping in and making conclusions about the condition of the horses (which of course they had never seen), the personality of their owner, and all kinds of other things. I am so glad that this place isn't like that.