How about use her street smarts? Things like avoiding situations like that, things like being assertive, using her voice?

Do you know why female police officers in the UK don't have guns? Because their voice and assertiveness is their most powerful weapon.

From a meta analysis of sexual assault studies: "One type of resistance, nonphysical, had a preventive effect on the risk of injury, while another, physical, in some cases elevated that risk." and "... prior studies have found that the type of self protective behavior used (i.e., forceful, nonforceful) may differentially affect injury risk. For example, Marchbanks et al. found that female victims of attempted or completed rape who engaged only in nonforceful resistance did not increase their risk of injury, but rape victims who used only forceful resistance or both forceful and nonforceful resistance were at increased risk of injury." [1]

Don't get me wrong, your hypothetical rape situation is a shit situation to be in, but the more effective way to resist a physical or sexual assault is to engage in non-physical resistance.

Not by pulling a knife and increasing the risk you getting seriously injured by yourself or others.

[1] Thompson, M. P., Simon, T. R., Saltzman, L. E., and Mercy, J. A. (1999). Epidemiology of injuries among women following physical assaults: The role of self-protective behaviors. American Journal of Epidemiology 150(3): 235–244.