IT COULD BE ONE OF MAY THINGS BUT I WOULD CHECK THE FOLLOWING FIRST:
*Is its CND Shellac. There are other "shellac" products out there. unless they say CND they are FAKE.
*Is the tech qualified in Shellac?
*Is She using all the correct equipment, the lamp should say CND on it, no other lamp will properly cure Shellac causing breakdown of service.
*Are all the products CND?
*Is she buffing the nail before application? If YES the answer is she is not providing your friend with a genuine service.
*Has she given your friend homecare? Gloves, no picking and OIL OIL OIL?
*Is your friend following the advice?
*Does your friend have weak peely nails, this can affect longevity.
Check these out.
I work in a hairdressers and Shellac both ladies nails and they go 3 weeks.
Your friend should be getting much longer than this.
HTH XX