Geralt can choose to either give Sile a dignified death or to abandon her. If Sile is not present then the character entry on her will be completed after the quest as if she had been present and Geralt has given her a dignified death.

Should I let Sile die?

There's really no point to killing Sile since she is now exposed and on the run. I was tempted to let her die since she deserves it to some degree, but then she promises information on Yennefer. Given Yennefer's importance, letting her live is worth it. Save the dragon.

Is Sile de Tansarville bad?

Sheala herself is not particularly evil and distains politics or national involvement, yet she did play a heavy hand in the plot as she saw a need to. ... If Geralt saves Sheala then he and Yennefer later find her in a prison in Oxenfurt when they are looking for Margarita Laux-Antille, who got captured by King Radovid.