the stupid hair you are complaining about is my hair. i was always the one begging for the hair that belonged to the girl on magazine. i am only a student but this is what i know. there will always be girls like me b/c curly hair can not have the sexy "hair blowing in the wind" effect. recently i have learned this from a very good friend. give them a trim, odds are it is very needed. i use biolage shampoo and conditioner for thick coarse hair. use some matrix sleek look and some paul mitchel super skinny, blow dry with lots of precession and a flat brush then pick up a chi straightning iron and work your magic. it's beautiful. she won't do it everyday so get her to try some gel and big sexy hair root lifter then blow dry it. she can do that at home and it will bring life back to her curls.  then she can have the best of both world and you are a genius. 

If you strongly believe that your clients hair won't do the style that they want, then my suggestion is, don't do it. Your integrity as a stylist should mean more, than someone who will do anything her clients want her to do.  As stylists, we also have an obligation to be educators to our clients. We need to let them know that if they want a particular style, and seeing as how their hair won't naturally go into that style, then they will need to do the work to get the style that they desire. If they are not willing to do the work, then you shouldn't do the hair cut.  You could also recommend that if they really want straight hair, that they could invest in getting a chi straingtening service.  This can be an expensive service depending on how long their hair is, but it is well worth it, since the hair is basically chemicall straightened using the chi iron, the only hair that ends up being curly is the new hair growth, which the client can straighted with a chi flat iron.  I have seen a client of the salon have this service done, and it is  5 months later and it is still straight. 

I hope this helps. 

I do relaxers all the time for people with this texture hair. You don't even have to relax it straight if they want the versatility of their curl. You can do a relaxer just to reduce bulk but keep the curl. It won't be as curly as before but curly enough.

A relaxer doesn't eliminate styling but does cut the styling time in half. Have them buy a flat iron for straight days and a diffuser for curly days. Also they should have the right product for curly days and the right product for straight days. More money in your pocket and a client who thinks you are a god. If this is too much of an investment for your client that's ok too. Just expalin to them they don't have the right hair for the cut and why, then show them pictures of haircuts that will work for them. You are still a god because you gave them the "only haircut that has ever worked for their hair" but also you have kept them as a client. You are the expert, prove it to your client.