What must I never do for the player as a caddie?

A caddie is not allowed to:

• concede the next stroke, a hole or the match, or agree the match score with the opponent,
• replace the player's ball, unless the caddie was the one who lifted or moved it,
• drop or place a ball when the player takes relief, or
• decide to take relief for the player, for example declaring the ball unplayable or taking relief from an abnormal course condition or penalty area. You may advise the player to take relief, but the player must decide.

Source: Rules of Golf (2023), Rule 10.3b(3).