Here is a blog post from UserTesting.com which had inconclusive results, with some recommendations for further testing.
First, serifs alter the outline of each letter, so they can be more difficult to make out for people who have dyslexia or visual disabilities.
Second, because those little horizontal lines are so small, they tend to reproduce poorly on older computer screens, which have much lower resolutions than print. (Retina screens on smartphones, however, have higher resolutions that make serifs easier to see.)
Here is one from NNGroup also concluding no strong benefits:
I think to move forward, you could conduct a simple preference test asking users which one they prefer and why. You can set one up easily with Usability Hub - https://usabilityhub.com/product
This way you can approach your client with a strong case based on user opinion rather than yours or the client’s.
My inclination is also that Sans should be more preferable since Serif is associated more for physical prints (newspaper) and users like consistent patterns. So much so that the Serif might give a negative “old” or “broken” or “product in beta” impression, affecting brand perception.
If nothing else works, propose an A/B testing and see which one performs better. Maybe your users are older gentlemen or lawyers who do prefer Serif?