How to help Philippa Eilhart?

Solution 1:

I was in the same predicament (Triss left Novigrad already, and Yennifer's Last Wish quest also done), but was able to find a solution.

Basically, you sit on the quest until the main story takes you to Kaer Morhren, after which you can speak to Yennifer and have her look at and hold onto the crystal. Then talk to Radovid to complete the quest.

Solution 2:

I'm happy to say my fear was premature. You will encounter Philippa much later in Novigrad in a main quest called Blindingly Obvious (suggested level 28).

(I'm not sure whether the previous decisions somehow affect the outcomes, however.)