I would ask her for the frame depth. It would not be hard for her to just measure one of her frames. The terminology you use is not standard for most Langstroth beekeepers, and I have never heard the term 3/4 Langstroth frame.
Once you have her exact measurement, you can use Michael's measurements above to see whether her frames will fit your box.
I would still ask her to measure her frames (most beekeepers have big stocks of empty frames, so it is easy to measure them!), but I just found this Swedish article:
It seems to confirm Michael's suspicion that 3/4 Langstroth is what we would call a "medium" Langstroth box in the US.
If you confirm that, her frames will not be deep enough for your Flow hive brood box. If you put them in you Flow brood box anyway, the bees would likely build "crazy comb" to fill the gap at the bottom of the frame. This is a bad thing.
I don't know where in the world you are, as your profile doesn't say, but you potentially have a few choices:
1. Buy an 8-frame medium depth Langstroth box. They are readily available in the US, but harder to find elsewhere. This can then be positioned underneath your Flow deep brood box.
2. Cut down your Flow brood box to size, as Michael suggests. However, I don't think anywhere in the world would run a hive on a single medium brood box, so you are going to need to buy another medium or deep to use for brood.
3. Find another supplier who uses deep Langstroth frames.
4. Ask her or find another supplier who is willing to make you a "package" of bees. This is usually done by shaking bees off frames from an established hive. They can then be housed in a new hive with a fresh queen. They are very common in the US, and becoming more common elsewhere in the world, so you might find this is a better option.
Hope you find a solution!