If the client could keep a set on for 3 weeks then you are applying the product correctly for her nail type so probably not much else you can do at your end.
Try to work out exactly what is making the gel lift - chemicals, hand sanitiser, products with alcohol in them, not wearing gloves etc.
Maybe there is something coming out from the cut flower stalks that affects the gel adhesion? Perhaps someone on here knows about such things?...
My best guess would be that she is not wearing gloves when handling the flowers and something is getting in around the edge of the gel and so the bond dissolves and lifts the gel. Hand sanitiser definitely does this so there are probably other chemicals out there that also affect nails this way.
Anyway, not your fault, she needs to adjust her work activities if she wants to wear gel polish. Her choice. You can only advise.