I would start by digging down level with the bottom of the footing on the down hill side, an area big enough to work. Then I would wrap the down hill side of the column and the two sides next to it with osb. 2x4s reinforcing the corners. Rig a strong ratchet strap from around 2/3s up on the leaning column to the base of the other column with enough tension to hold leaning column in place. Brace the column with a couple of 2x4s from above the ratchet straps to strong stakes in the ground on the down hill side. Space the stakes around twice the width of the column apart and around 8 plus fleet from the column. Dig or drill a hole on the down hill side centered on the footing about a third of the width of the footing and three to four feet deep. Fill hole with fast setting concrete. It needs to be under the footing enough to be able to use a jack between it and the footing. If the footing is round on the bottom you will need to cut a inverted shelf for a jack point. Next day use a bottle jack to level column and Re-Tension the ratchet straps and adjust the 2x4s. Dig a hole on each side of the previous one and fill with concrete to the bottom of the footing. Let set, remove the jack and fill that space with concrete