Ambulance Alerting Device
I am not aware of any type of alerting device that would perform this particular function. However, the efficacy of cochlear implants for the remediation of severe-to-profound hearing impairment is well accepted. You stated that your patient functions ''quite well'' with binaural digital hearing aids and lip-reading combined, and that is a common finding. However, it is impossible to ''lip-read'' the wail of an approaching ambulance siren!
Importantly, FDA guidelines do allow implantation in patients who receive some benefit from hearing aids. A cochlear implant evaluation takes into consideration whether the implant will provide more sound and better hearing than she currently receives through her residual hearing and hearing aids. Environmental sounds are important - especially in real life situations such as the one you described in your question and with respect to other warning sounds and signals as well.
No one can predict how much benefit a particular person will receive from a cochlear implant. Several factors which appear to contribute to success include auditory memory, duration of deafness, status of the cochlea, motivation and attitude, as well as participation in rehabilitation programs. Research has shown that prelingual individuals can derive substantial benefit from cochlear implants, including an awareness of environmental sounds (warning signals) as well as enhancing lip-reading abilities. For the safety of your patient and those traveling on the road along side of her, cochlear implantation may be an option to be considered.