Bob through his actions is harming Alice. Thus Eve may use the minimum necessary force to help Alice as Nothilfe, which is defined as using §32 StGB (Self protection/Notwehr) and §34 StGB (justifying state of emergency/Rechtfertigender Notstand). As a result, it is allowed to harm someone to the degree of self protection for the benefit of someone else that can't protect themselves - such as Alice. However, the amount of force allowed must be proportional to the harm done to Alice - so Eve may use the least needed amount of force to get Bob from stopping Eve from helping Alice.
As an example, Eve might shove Bob out of the way, hit him, or use the threat of serious harm (which is usually illegal!) to deter Bob from getting in the way, but unless Alice is actually at risk of dying from Bob keeping her from applying pressure on a lacerated artery this very moment, she can't shoot at Bob - that would overstep the Notwehr as, but might not be punished if the overstepping is for the right reasons defined in §33 StGB.
Bob also is liable for not rendering aid, §323c StGB unterlassene Hilfeleistung/Behinderung von hilfeleistenden Personen (Failure to provide assistance/hindering persons providing assistance).