Perhaps it is kufr in his case, but there's a probability that this friend of his has the wrong understanding of what agnostic means. This question shouldn't have been asked by him nor should it be answered IMO. Allahu 'alam
https://seekersguidance.org/answers/islamic-belief/did-my-confused-friend-leave-islam/ See the foolish response to the above question. Clearly, if someone claims to be half agnostic, this is the same as claiming he is half hindu or the like- which is clear cut kufr. But the alleged 'shaykh' disagrees. Does have anyone have relevant references to rebut the answer in the link?