Roz, we're in a retail business, and our first responsibility is to provide our customers what they want. I operate under the assumption that they want to know the consequences of bad framing decisions.
If she is adamant about framing a serigraph without glazing, my strategy would be to verify that she understands how that limits the useful life of the art and assures its destruction eventually, or immediately in the event of a mishap. Do whatever it takes to help her make a well-informed decision.
If she ignores or discounts your advice thinking you just want a bigger sale, the purpose of educating her would not be served. In that case I suggest going through it again until she gets it. Having her sign a disclaimer might make her think twice now about ordering inappropriate framing, but it would not save you in court later.
If she pays attention to you, understands the hazards, and still wants no glazing, I suggest doing what she says for a fair and profitable price.