EFF/KRF: Inverted yield curves are often observed at the front end of recessions. But there's no evidence that they predict stock returns, which also tend to predict (decline in advance of) recessions. Your client's implicit premise is that bond market investors predict future economic activity better than stock market investors. The evidence says that both markets are moderately good at predicting future economic activity, and inverted yield curves are not reliable predictors of stock returns.
Eugene Fama and Ken French are members of the Board of Directors for and provide consulting services to Dimensional Fund Advisors LP.