Here is another answer, providing an in-game + meta-game (by writing this I am contributing to its in-game premise) perspective:
So you ask whether a fiend can be redeemed. The answer is affirmative: yes, as long as you believe that it can happen. You see, the Outer Planes, are all about belief. Why do you think the Gray Waste is gloomy and Carceri is all deception? Why is the Beastlands so wild and beautiful? Because of you, of course, because of you, me, and millions of other clueless beings living their lives in pain, fear, desolation, hate, but also in love, hope, curiosity and awe. So all the Outer Planes are a reflection of the good things and bad things in our collective conscious, our total mental presence in this multiverse.
Beings like fiends and celestials, modrons and slaadi, are all reflections of the
"belief matter" of their respective native planes. Archons are these law-abiding pillars of good, why? Because they are Celestia personified. Why are yugoloths those selfish schemers that they are? Thanks to your selfishness empowering the Gray Waste. So you might conclude that they can never become something else. With pure good or pure evil personified, you would think that a deva is incorruptible, or a cornugon irredeemable, yeah? Wrong!
It is still all about belief. Do you like stories of fallen angels? Do you find them interesting, but more importantly do you find them believable? Pride, or jealousy, or the plain old feeling of boredom causes the powerful planetar to let go - you find that intriguing, don't you? If your answer is yes, congrats, you have allowed some celestials to loose their paths.
Likewise, if you give credence to stories of risen ultraloths, some will surely rise. If enough people believed that there could be succubus paladins, in this vast multiverse, there will be some. Sadly redeemed fiends are a lot fewer in number than fallen celestials, mainly because people like stories of falls more than stories of redemption.
Before I conclude, let me give you tip. If you fall in love with a particular succubus and expect it to return your love, or if you are asking for mercy from a molydeus, you are a hopeless fool. But when enough of us keep in our hearts a path for seeking atonement, a way to forgiveness and a belief that love and compassion can overwhelm evil, there will be risen fiends of all kinds in the vast expanse of the multiverse.