Your customers know a lot. You know that. They often know as much about your products as you do, and usually way more than the part-time neighborhood kid you hired to answer phones and wrap gifts.
Where did they get that info?
From that most informative and reliable source... say it all together...
What? You thought I was going to say Internet? Okay, yes, the Internet is where that information is held. But the information she tends to trust the most is reviews from other users. Her new best friends. People who claim to have used your products and are now experts on those products (even though they never read the instructions, even though they may have used the product in ways it was not designed, even though they may have a slant against or for certain brands, even though they have nothing to compare this product to, even though you have no idea if they are a shill for the brand.)
So let me ask you... Are you reading those reviews, too? Are you looking at what other people are saying about the products you sell? Are you finding out what the end-users believe is the downside of your product?
Your staff should, too.
Then when she brings it up, you'll be prepared. Or even better, you can talk about the downsides right up front. Not only will it reassure her that you know what you're talking about, it will reassure her that you understand both the pros and cons of the product and won't sell her anything unless she knows, too.
It is a great way to create trust with your customers. Plus, when she sees that you read the same reviews she read, she feels more of a personal connection to you. You're her type of person.
PS Trust is the most precious commodity you can build for your store. Own that trait and you'll own more than your share of the market. Talking willingly and openly about the downside of a product, is just one way to create trust. To learn more about how to build trust, read my friend Tom's book, Currencies That Build Credibility.