Tom,
She cannot be vaccinated. Her body does not respond to a vaccine the way one with a healthy immune system does. She has a condition called CVID which is a deficiency in two of the four immune globulins (in her case, IgG and IgD). This condition makes her especially susceptible to respiratory illnesses. The only reason she is able to have a functional immune system is due to infusions of IgG (one of the missing immune globulins) every three weeks. The only real benefit of this is that the source of IgG is plasma donations and these immune globulins she gets have already been "taught" about certain illnesses by their donor's immune systems. However, these immune globulins are only temporary, hence the 3-week interval. We are leaving the day after an infusion so her immune system will be at its strongest at the beginning of the trip. I will be shocked if she isn't at least a little ill soon after we return.
I am a biologist so I recognize that you are correct, a standard surgical mask will do nothing for her however, one with a higher level of filtration, such as those used by people using volitile chemicals or fine particulates, should provide at least some protection for her.