I know for me personally I do the same thing. I have close to 100 different colors in colored powder. I dont always want to do my own nails and back bills by hand I just dont like doing at all. So I bring in my own color (a lot of places do not have colored acrylic even though it takes the same amount of time as p&ws). I dont ever expect to pay less.
Honestly I would do the same with tips. If I had a tip style say the deep smile lines that I knew the tech didnt have then yes I would bring them in, but no I would never expect to pay less.
Have the client sign a waiver that way if something goes wrong, but think of all the different types of tips we all use as it is. No well, half well, full well, some prefer theirs thicker some thinner.
Next, she'll want to buy her glue/nail cleanzer/files/acrylic/gel/polish all at boots or whatever, and have you put her nails on for free... never mind your time, advertising expenses, school expenses, rent, etc etc etc...................................................
NO NO NO NO NO
If she wants to use her own tips, let her do her nails herself.
(PS: in any case, what would you deduct? Pennies? It makes no sense. Tips are not where the cost lies. The cost is incurred by your time/expertise, acrylic/gel etc... NOT in tips. If she argues, say "fine, but you'll only save 1 penny a nail")