Its best not to enagage at all and just show that child love. “Tough Love” will likley fail and estrange you from the kid. Best hope is to continue to treat the child with love (and Boundaries like not breaking shabbos in house or bringing in Non-Kosher food..But most would likely accept that) and hope whatever turned them off will get lost
Giving them shiurum will likley not work either, if the person hates torah and learning, giving them more torah and learning isnt going to work