English speaking financial industry jobs in Frankfurt

I guess you would need advice from a real insider. IT and a few other niche markets seem to recruit people who speak English but not German.  But I would doubt there is much demand in the financial/banking sector. Most banks have been downsizing over the last years and tens of thousands of people have been let go. And maybe in the upper echelons of an international group there might be some such positions but these would seem to be rare.  Anyone in such a field in Germany will speak good English so it’s not like they need someone with English skill to be brought in. Why would they bother with someone who doesn’t speak German? I can only suggest that to get a professional job then she might have to be creative and think of jobs where her skills are still relevant although it is not necessarily the same as what she has been doing. Assuming you are on a blue card, she should be able to accompany you and have the right to work. Thus instead of getting a job and work visa beforehand, she could settle in and ask for assistance in finding a job at the unemployment office (Arbeitsamt).

While Frankfurt is the finance capital of Germany, it is of course much smaller than London. But it may benefit from Brexit by mopping up some of the organisations moving out from there.
As Tom already mentions above, German language skills will be her key to any job that requires communication with customers, partners and/or colleagues. Without them, she will have to search hard to find a purely international niche.