To my limited knowledge, the Canadian Conservation Institute's "CCI Notes" describe the most up-to-date methods and materials for framing art on canvas. You can buy individual sections of the publication.
Most conservators would echo the CCI recommendations. However, all of that comes with technical jargon and peripheral gobbledygook that your customer probably wouldn't understand -- and it may be irrelevent to your purpose, anyway.
Technical expertise is not required for your customer to understand the value of protecting the painted surface from:
1. soiling
2. airborne chemical contaminants
3. changes of temperature and humidity
4. accidental damage
The recent archives describe these and other attributes in nauseating detail.
When a reltively-uninformed customer insists on believing what he/she has heard, instead of what makes sense, then it is really a question of who is more credible. You just need to make a more sensible argument than the other guy.
For example, ask the customer why he/she doesn't want glass. The first answer will probably be, "I don't like the look/reflections/glare of glass". Nod knowingly, even though you know this person has paper art framed under glass. Then show him/her Museum Glass.
Then, when visual objections go away and price becomes the issue, point out that Museum Glass almost certainly would cost less than the first cleaning or repair.
Or, if you sense that this customer is more concerned about getting his/her way than doing what's best for the art, then just frame it naked, as specified.