You don't need to use any fixative of any kind on this, just put a mat on it and frame it behind glass. Nothing will happen to it, the glass will protect it. I have a painting on Sennelier LaCarte which is 13 years old and framed under glass. I never us a fixative. It's the same as it was when I originally painted it.
You are correct in your concern about fixative darkening the painting. Most fixatives will do that, the painting looses its sparkle. Pastels aren't as delicate as one might think, they're actually very resilient, and archival. I am thinking you've used Sennelier LaCarte paper here, which definitely cannot take moisture. It is an excellent paper, just not good with moisture. You've used excellent paper and pastels, which is the most important thing in ensuring its longevity. Frame it under glass and you are finished!
Beautiful work, I am sure your client will be thrilled.
Oh, and I forgot to mention, Welcome to the Pastel Forum!