The most relevant performance measure for Beesly's portfolio would be the Sharpe Ratio.
The Sharpe Ratio is a measure of risk-adjusted return, which considers both the return earned and the volatility (risk) associated with that return. It calculates the excess return per unit of risk (standard deviation).
Since Beesly promises investors a fixed 10% return regardless of the performance of any index, the relevant measure would be to assess the risk-adjusted return of her portfolio. The Sharpe Ratio will provide insights into how well she is generating returns relative to the risk taken.