Jamieson, Fausset and Brown on Hosea 4:17

17. Ephraim--the ten tribes. Judah was at this time not so given to idolatry as afterwards.

joined to--closely and voluntarily; identifying themselves with them as a whoremonger becomes one flesh with the harlot (Nu 25:3; 1Co 6:16, 17).

idols--The Hebrew means also "sorrows," "pains," implying the pain which idolatry brings on its votaries.

let him alone--Leave him to himself. Let him reap the fruits of his own perverse choice; his case is desperate; say nothing to him (compare Jr 7:16). Here Ho 4:15 shows the address is to Judah, to avoid the contagion of Israel's bad example. He is bent on his own ruin; leave him to his fate, lest, instead of saving him, thou fall thyself (Is 48:20; Jr 50:8; 51:6, 45; 2Co 6:17).