Call your supplier and see what they suggest, some solutions can be diluted with water for a less intense color but its best you ask the supplier if it is possible with the solution you use. Otherwise buy in a trial size of a lighter solution, I have seen 5% before, there are probably a variety out there by now.
I agree with Trinitia, the gun setting wont make a difference and washing off sooner would probably help but then again some solutions still go as dark as usual but just dont last as long if you wash off sooner. You could try the express ones and wash off after half hour but will she remember to wash it off in time? In my experience, customers dont always remember and then blame you after. You cant exactly prove when she washed it off can you
Good luck