I think the fact they dated literally years before you even knew the fellow changes everything. I don't believe you need to "ask permission". I believe all that would be needed is to mention in passing that you are planning to pursueing a romantic interlude with said lady to see where it might lead. If anyone would need to "explain" anything it would be said lady because as you stated she is closer to him than you. And her explaining would need to be something along the lines of "we was years ago so get over it", in a nice polite way of course.
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