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Can you go there before the shoot and run a series of test shots to get a feel for what your tools will and won't do for you in that environment (i.e. ceiling height/wall color/etc.)?
Much will depend on the kinda of "vibe" you are trying to project as to how your balance your flash / ambient. One problem with interiors is the falloff from a single flash, particularly if it is on camera. If you can get your flash off camera, or bounce (wall,ceiling/reflector), you can offset some of that fall off look, or you can use it for dramatic rendering. Of course, if the walls are dark,ceilings are high and far away, it'll "eat" your light as well.
Does she want a "crisp & clean" look, or does she want a "we get down to business" look? Make some test shots with both direct flash vs. bounce/reflected and you'll see the range of what you can do with your single flash. Also, it will give you an idea how much you can vary your shutter/aperture combinations to get some balance or weighting to the different areas in the room relative to the available ambient.
If you are working on mostly portraiture (people/equipment), you could easily bring along a reflector (foamcore, etc.) to bounce your flash into. You should have plenty of light for close distances ... it'll mostly be an issue of "falloff" management and choices @ hard/soft lighting. Also, note that gym equipment may have plenty of chrome for specular reflections that can fool metering a bit or be otherwise distracting.
Test shots (subject not necessary) highly recommended before the shoot to study what you've got to work with ... if you're not already comfortable with doing it "on the fly".