I agree with Lolly that if they have broken it is probably just down to the amount of growth in 4 weeks, which is considerable.
As Jacquelineanna says, it is a 14 day manicure . . . anyone getting over that should be highly delighted as they are on a "bonus" lol
But not to the point where they just let it go on and on when the length is far too long (for them). She must have begun to see the signs . . . they don't all break/chip all at once with normal use. Once over the 14 days, at the first signs of any chipping at all, the client should book in (in an ideal scenario they would have their appointment already booked but we all know what they're like sometimes).
This is definitely not your fault, maybe you are relatively new to nails? You need to reinforce the idea that it is a 14 day thing at the time of application and remind her when she does come in that she has left it twice the recommended time and so what can she expect really?
I have had several clients go 4, 4.5 even 5 weeks and return in perfect order apart from the growth, but the factors that influence this are that a) they are not particularly "heavy" on their nails and b) they have a pretty strong natural nail underneath. And of course they should be using Solar oil . . . although my sister in law went 4.5 perfect weeks and completely forgot to use it at all
Shame on her! But she has very strong nails.
Btw, if it's part of a deal, by all means give away a pinkie of Solar Oil . . . but don't then feel duty bound to supply them with it for free whenever they run out! If you feel awkward, you could try saying something at the time you give it, like "when you finish that bottle, I've plenty more in stock and it's only £3.50 or whatever it is" - then that sets the tone from the outset. hth's xx