Hmmm, if I were to imagine myself in my sixties, after going through the menopause etc and possibly not being that satisfied in the bedroom for a while (you mention another man could do things you can't?) I can't blame her for not being that particularly interested. After going through the menopause your hormones changes and that can result in a lower sex drive and don't forget your age. Is she stressed at the moment? Worried? As yourself all these things before mentioning another man to her. That may really hurt her feelings. If it has just been you and her for years and years it could really mess up your relationship... Adding someone else into the sexual relationship could either prove bad or good but if she thoroughly enjoys this other man she may not want to even have sex with you anymore... Think of that. Many many things to consider. Don't wreck your relationship for better sex because let's face it you are still having sex and that is a mighty good thing at your age...
Everything happens for a reason. Live for the moment and have no regrets.
There are good councilors who specialize in couples sexual issues, that might be the best route to take. That in cooperation with a medical doctor keeping an eye on hormone levels could make a world of difference. I doubt just bringing another guy into the picture will help. Unless of course he's 25, hung like a horse and has a PhD in psychology...
Talk, talk and talk some more, you can't turn back the clock, but reliving adventures from the past can be fun at almost any age. Try and show here that she's only as old as she feels... And that she doesn't feel old to you. You never know what you might awaken.