Who art thou that judgest another man's servant? (observe the emphatic position of σὺ) to his own lord he standeth or falleth. Yea, he shall be made to stand: for the Lord (better supported than God, as in the Textus Receptus) is able (or, has power) to make him stand.
The standing or falling here spoken of may be taken to mean standing firm in, or falling from, a state of grace (cf. Romans 11:20, Romans 11:22), rather than acceptance or rejection at the last judgment.
"For God is able," etc., seems to require this meaning. The non-abstainer's freedom does not endanger his position; for God is powerful to sustain him, and to God alone he is accountable.