Firstly, "silny" :) No ł here.
Secondly, as I understand that the drink is 'too strong', actually the word "silny" doesn't work here anymore. It should be "mocny". Or "mocna" if the drink's name is feminine, which seems more common.
Thirdly, it should be "dla niej". Yes "jej" is a Genitive form and "dla" takes Genitive, but some pronouns have this special n-form and it's obligatory after a preposition. So "for her" = "dla niej".