Chemical sensitivity is still pretty rare, so it might be overdoing things to ask every customer about it. I think it reasonable to expect someone with an allergy to tell you rather than hope it will be OK. Can you imagine eating a bunch of homemade cookies and then announcing your peanut allergy when the paramedics are loading you into the ambulance?
Question #1: How do you know she's sensitive to anything you used?
In the absence of science many people decide what they're going to believe and arrange facts to suit.
If a future customer mentions chemical sensitivity (and you aren't using some off-brand nuclear/oxidizing/encap/ozone crap) just offer to clean a piece of loose carpet that they can put next to their chair or bed. If they have no reaction your stuff is probably going to be OK to use. If they have a reaction they can toss the carpet outside and you can decide whether you want to change chemistry or decline to clean for them.
Question #2: Not to ignore those nasty chemicals, but do your choice of cleaning products have a nice, fresh-n-fruity scent?
It's been my experience that most chemical sensitivities have more to do with fragrance than cleaning related chemistry.
We use products with no added scent, even if a lot of customers might like a "pleasant" odor. When customers ask, I will tell them that we clean for a lot of people that have chemical sensitivities and those people usually find they can breathe easier after we clean. I then offer to do the loose-lay clean for them. Since we've been made aware of the issue, only two customers have requested water-only cleaning.