Hi,
I imagine what your farrier saw(I'm *assuming* he does know what he's on about) is a prominent 'growth' or 'laminitis' ring. This does indicate that there was some metabolic or mechanical upset at that time which inflamed the laminae & damaged it at that point. If it's only one ring(per foot) and has nearly grown out, chances are it was a 'one off' type event - could be diet related, stress, illness... - that was short lived & over many months back. As you say it's only on one foot, I'd guess it was mild but there may be a mechanical issue too/instead - is that foot steeper, for eg? I would not call that 'founder', but many use the terms interchangeably, don't differentiate about degree, etc. This sort of thing may be called 'sub clinical' or 'low grade' laminitis. Depends on other specifics & signs I reckon, as to whether it's any worry or any cause to change what you're doing IMO.
You said the horse has bad feet, which can mean anything really, so something there may have given your farrier more to go on considering what he said. So with so little info from you, this is sort of my generic opinions on the various issues - food for thought for you...
As for diet & supps, a good quality nutritional supp is a good move IMO, as domestic horse's diets are generally deficient or imbalanced in a range of nutrients. 'Hoof specific' supps may or may not be well balanced & have everything the horse needs. FeedXL.com is one good source IMO to sort out all that.
As for supps to 'cure' laminitis, if it's an insulin resistance problem, cinnamon has actually been scientifically shown to increase sensitivity to insulin. There is also 'Equisure' which is a slow release magnesium, which can reduce hind gut acidosis which can cause laminitis - acidosis is generally related to too starchy feed. There's also 'Founderguard' which is an antibiotic, for treatment of the same prob. I don't think feeding antibiotics is a great idea, but many do. There are other herbs & nutrients which have not been fully tested, but there is anecdotal evidence to suggest they may help.
When you say your horse is not on rich grass but gets good hay, it's worth keeping in mind that grass doesn't lose sugar content after cutting & storing, so it depends on the quality of the grass & when it was cut, etc as to how rich your hay is. Also overgrazed or drought stressed grass is generally higher in sugar content. Safe Choice does have corn, wheat & molasses in it, so I would personally consider something 'safer' if your horse needs extra energy of hard feed. If you are going to feed starchy feeds, it's especially important to ensure you feed it little & often with lots of roughage, rather than just one or 2 larger meals daily.