John Gill Commentary 1 Samuel 28:17

"And Jehovah hath done unto thee, as he spake by me: and Jehovah hath rent the kingdom out of thy hand, and given it to thy neighbor, even to David." — 1 Samuel 28:17 (ASV)

And the Lord has done to him
To David, Saul's enemy, as he insinuated he was:

as he spoke by me ;
pretending to be the true Samuel, and wearing the guise of him, he speaks his very words, which he was well acquainted with, and could deliver exactly as he did:

for the Lord has rent the kingdom out of your hand, and given it to
your neighbour ;
which words are expressed by Samuel, (1 Samuel 15:28) ;

[even] unto David ;
which is added by the apparition, by way of explanation, interpreting the words of David; which he might safely venture to do, seeing such a train of circumstances had occurred since the delivery of these words, which plainly made it appear he was intended.