How could I convince a client that visually impaired customers might want to use his e-commerce website? I want to demonstrate the importance of accessibility and convince him that we should try to make it accessible so that people of all abilities can use the site. He thinks visually impaired users aren't in his audience, and if they were, they would ask for assistance from a sighted person.
Chris has raised a very valid point that you need to consider that your potential user base could contain blind users and by effectively alienating them you are also losing business.
However there is also the possibility of bad press since you might end up alienating the friends/relatives of those blind users and general loss in reputation.I am not sure which country your client is located in but if you do not make your site accessible you might be violating the disabilities act in your user bases' country. For example, Target had to settle a 6 million dollar lawsuit because for years it didn't make it site accessible
To quote the article
Since 2006, the National Federation of the Blind had pursued charges that Target had “failed and refused” to make its web site accessible to the blind and thus violated the Americans with Disabilities Act as well as two California civil rights statutes: the California Unruh Civil Rights Act and the California Disabled Persons Act.
But in a settlement reached on Aug. 27 between Target, No. 19 in the Internet Retailer Top 500 Guide, and the federation, Target will establish a $6 million settlement fund from which litigants in the California lawsuit can make claims. The National Federation of the Blind will also certify Target.com through a non-visual accessibility web certification program once planned improvements are completed in early 2009. Target and the federation also have agreed to a three-year relationship during which the National Federation of the Blind will perform accessibility testing of Target.com.
As I said, I dont know the country where your client is located but you do need to consider that the number of blind people is not steady number and unfortunately there are more and more people who lose their sight every year. As per this article
More than 1.5 million blind or visually impaired people have access to the Internet, according to the U.S. Census Bureau. And every year, 75,000 more Americans become blind, according to the National Federation of the Blind.
So effectively you could not only be losing out on a significant chunk of the existing population but could also lose out a future subset of the population who have unfortunately lost their site.
You would need to actually show him someone using a site setup correctly and then failing to use his site. However, this might not be possible, or he could choose not to believe the evidence anyway. You need to explain the situation in terms he understands - money.
Show him the statistics for how many blind and visually impaired people there are in your country. Then explain that if people can't use his site to actually buy stuff unaided they will leave and go to a competitors site.
This will reduce his potential customer base and hence sales by that percentage. This will have an impact on his profitability.
The more people who can use this site, the greater his profits will be.
When I've done user testing with blind people I've found that resolving issues for them would make the site better for everyone.
Another point to make is that search engines a website in much the same way as a visually impaired person does. So an accessible site is a more SEO friendly site
WebAIM is always useful for information - http://webaim.org/articles/visual/blind
As mentioned by someone else there's likely to be a legal requirement for your clients site to be accessible.
Install a screen reader (free one could fit). Turn the screen off. Navigate and show him what the experience would be. For a better effect, if you know someone who's blind, make him meet your client and let him explain how frustrating it is to try to buy stuff online and you can't because of poorly coded or designed website.
Also I like to bring statistics and show them how many customers they could lost for this. Accessibility is not always for blind people. Think about your grand-mother who uses a high contrast screen and increase the font size.
I once told a client I was color blind and I couldn't tell the difference between new hot info (represented with red text) and archived info (in blue). I lied...but it was efficient, because if you say : Some people might not see it..they don't care. If they have the read deal in front of them..it's another story.