maybe she can see value in herself

I would do exactly the same to a chick because i know i have a LOT of value

by telling her you were out with another girl (which she will take as you fucking her just like you want to do with Jess) she will feel like she has no value to you and wether she sees you as just a fuck friend or wether she sees you as somthing she might want to pursue further - she would rather walk away