Lyrisa can quickly dismount as a simple action (since she is sitting on the back of the horse rather than riding it fully. (dismounting from an animal you are riding is a 'major' action unless you try to dismount quickly, which takes a Riding Roll (DL 25)). 

Lyrisa, however, is just sitting on the back behind Leifr, so she can just hop off quickly rather than dismounting.