It is a sad state of affairs when people find themselves in a situation where they are offering free to a good home, I find it very sad that old horses/ponies are put up for sale.
I am personally very lucky that OH and I are in jobs where we will always be needed. I know that we can offer homes for life. Any horse that comes to me has a home until its end of days.
I think that if a person ever finds themselves with an old horse with problems that they can no longer afford to look after, as the horse deserves to be looked after, then the kindest thing is to allow the horse to pass on.
If the older horse is surplus to requirements but is otherwise healthy then i would propose a sharer or even a permanent loan to the right home (better if the horse can stay on current yard).