We're speculating about talking the customer into doing things he said he doesn't want.
There's no doubt that glazing, matting, and framing would be the right way to go, but in order to help him understand why, it is necessary to know the motivation for his specifications. Why does the customer want no frame? Is it a matter of money? A matter of appearance? A matter of size? What are the real reasons behind his specifications?
We always try to give customers what they want, but what they SAY they want can only be based on what they perceive to be true -- which is sometimes dead wrong. After we inform them about what they are buying, their wants often improve as they begin to make informed choices.
The worst case here would be to try to mount the art, as asked by an uninformed customer, and do irreversible damage for which the well-intentioned framer would be blamed.
The second-worst case would be to successfully mount the art, and have the customer realize later that what he demanded reduced the visual and sentimental values of the art, as well as its monetary value.
If you haven't attempted the mounting yet, then it is not too late to decline the job. I would tell him I have changed my mind after further consideration, make the recommendations I know are right, and let him decide whether it will be my way or the no way.