They helped them not, i.e. the Manassites here named, and the rest of David's forces, to whom they had now joined themselves, did not help the Philistines in battle, as David had pretended to do.
Related Commentaries of 1 Chronicles 12:19
1 Chronicles 12:19
19 And there fell some of Manasseh to David, when he came with the Philistines against Saul to battle: but they helped them not: for the lords of the Philistines upon advisement sent him away, saying, He will fall to his master Saul to the jeopardy of our heads.