I'm assuming that the nasally sound is not the sound you are looking for here... The best fix for that is to sing from the diaphragm.
Sometimes this type of voice is just what a song needs, more often though the singer just needs to learn how to use their lungs to pump some serious air through their whole head, not just a little through their nose. That makes a lot more difference than any recording technique. It's about projecting sound and emotion.
If you like the sound but are having difficulty recording it, I'd suggest getting up close on a large diaphragm condenser, probably using some compression/expansion and equalization to help bring out the consonants and mouth sounds which tend to be indistinct with nasal voices. I'd start by finding and applying some cut to the problem frequencies that are causing mud or fuzziness (narrow band eq, set to cut, sweep around till you locate them). Then look at boosting other frequencies to improve clarity if need be. It would help to know if this was a sound you are trying to celebrate or work around.
I think it will depend on how nasily they are but I wouldn't put all your hope on a ribbon mic. It is not a bad idea, but I think it is reaching slightly.
I know it offers no help whatsoever for all of us to say things like "get another singer" etc, but again, studio equipment is a tool and tools have limits as to what they can fix.
Basically, your vocal is vital to leading your track, and if you have to make these kind of changes to the vocal, just to make it sound normal, I would be concerned that this vocal is going to be good enough, to stand out in front and lead your track. There is nowhere for a lead vocal to hide, within a track that it needs to lead.
Without knowing more about your particular set up it's really hard to give you an accurate answer.
A nasal sound could certainly be due to a bad mic cable or improper input selection as much as it being the singer. Could be a bad mic. As a rule of thumb I never use condenser mics on nasal sounding singers.
A bad impedance mismatch, too much eq, too much compression, terrible recording room, cheezy mic preamp, degraded mic, all of these things can over emphasize the nasal characteristics of a singer.
If the singer is physically nasal sounding, write some new material that makes him the next big thing or consider voice training (which can really make a huge difference).
Tell us what you have to record with, and how you place the mic relative to the singer and perhaps we could give you better suggestions about how to improve your tone.
Yes, you can't really fix someones voice. I mean you can fix pitch and some other flaws, but nasal voice... Probably nothing can help that. Plus, nasal singing is not the correct form of singing... At least in the known genres... Lol give more time to practice to your vocalist.
For me it's usually picking a microphone that complements the singer's voice, maybe highlighting other aspects of it that I like, or that set them apart from other singers. I really don't mind a nasal vocal that much as long as the performance is compelling. Every singer is unique and I like the character of their voice to show through in the recording. I know this probably doesn't answer your question, but maybe it's a useful alternative to consider. Hope this helps.
p.s. Check out Italian artist Eros Ramazzotti for a vocal with a nasal quality if you haven't heard of him. He's very popular in many parts of the world and his identifiable voice is perhaps a big part of what made him so successful.
The upside is that as long as the singer is willing to accept some simple coaching, singing from the diaphragm is fairly easy to learn and can be accomplished fairly quickly even if self taught. It's not nearly as difficult and learning to sing on key or correct a lifetime of bad habits in phrasing or ornamentation. It won't turn them into an instant master vocalist, but the basics of breathing and projecting will go a long way and make an order of magnitude improvement in the singer's results.
Push from the gut, sing from the throat, pay special attention to diction and clearly articulating rather than slurring over the consonants. Be sure to show them the results when they do well and stop them when they get fatigued or fall back into bad habits. The singer will quickly see the benefits and begin to work to achieve them on their own. My guess is they will show noticeable improvement right away.
A slightly "dirty trick" is to turn their vocals down a notch and make them sing louder to hear themselves over the backing tracks. Sometimes this will just tick them off, but it can work well if you let them know what you are doing up front and don't over do it...