Just curious to know if this is a significant factor. That is, will a brilliant female vocal trainer be able to figure out and help a struggling male student just as efficient as a male vocal trainer?
Will a brilliant grown-up baseball trainer be able to figure out and help a struggling kid student just as efficient as a 10-year old baseball trainer could?
Sounds a bit absurd, doesn't it? The teaching skills and knowledge and adaptability are way more important than bodily similarities of teacher and student. You won't find student and teacher with exactly matched physique anyway, so if you have a teacher who imagines if you blindly do everything like he does, the same results will ensue, you are likely to be disappointed.
Good violins are not CNC-milled but hand-crafted from the raw materials because every block of wood is different, and if you want to get the most pliable cover without it breaking apart, you have to work with the grain.
The equivalent in teaching is teaching classes as compared to individual teaching. It may make sense to separate different vocal fachs into different classes. But there is no reason that a good teacher for a particular fach needs to come from the fach even in group settings. He or she needs to be good at working with what the student brings in, and with what he develops in the course of the lessons.
In the context of classes, it may be argued that the reduced individual feedback makes it easier for the teacher to generalize if he or she has him or herself as an example to work from.
But for individual lessons, the student's current capabilities are paramount for determining the progress. And it is much more important that you have a teacher that understands this and is able to work with that than it is that the teacher shares some bodily traits with you.
Apart from having to sing an octave (or two) out, it's not going to cause problems. Voice production and projection are the same for each sex. The choice of key may cause slight problems, depending on the two tessituras, but a good teacher will cope easily with that.
The head of music in the city I used to work always sung in falsetto when teaching junior choirs. He's the only person I've ever heard do this, but he was convinced it helped. I'm quite sure it didn't!
The teacher should known, especially is she is "brilliant" (I interpret as "successful and experienced"). Ask her and if you see or hear any doubts immediately, or after the first lesson, do not insist.
This may depend on the vocal ranges of both student and teacher. Men and women have different voice types but the ranges may overlap.