Are these the semi permanent eyelash extensions?
If so I def would not be giving out glue to clients, they are not trained in applying eyelashes and the glue that we use for semi permanent lashes is alot diff to the glue that is used for strip and cluster lashes, its medical grade for starters. The lashes are bonded onto the lash not to the skin, the client will not know how to do this and may end up causing damage to her lashes. This is just my personal opinion, also if they are applied correctly they should not just fall off.
If you are applying cluster lashes/ strip lashes/ individual false lashes she can buy them from boots anyway so can get the glue there.