There's a rule for that...
22/6 Competitor Requests That Ball in Position to Assist Him Not Be Lifted
Q. In stroke play, BGÇÖs ball lies just off the putting green. AGÇÖs ball lies near the hole in a position to serve as a backstop for BGÇÖs ball. B requests A not to lift his ball. Is such a request proper?
A. No. If A and B agree not to lift a ball that might assist B, both players are disqualified under Rule 22-1. (Revised)
22-1. Ball Assisting Play
Except when a ball is in motion, if a player considers that a ball might assist any other player, he may:
a. Lift the ball if it is his ball, or
b. Have any other ball lifted.
A ball lifted under this Rule must be replaced (see Rule 20-3). The ball must not be cleaned, unless it lies on the putting green (see Rule 21).
In stroke play, a player required to lift his ball may play first rather than lift the ball.
In stroke play, if the Committee determines that competitors have agreed not to lift a ball that might assist any competitor, they are disqualified.
30-3f/11 Request to Lift Ball That Might Assist Partner Not Honored
Q. A and B are playing C and D in a four-ball match. BGÇÖs ball is near the hole in a position to serve as a backstop for AGÇÖs ball. C requests B to lift his ball. B does not comply and A putts. What is the ruling?
A. B is disqualified for the hole for failing to comply with Rule 22-1. If AGÇÖs ball strikes BGÇÖs ball, A would also be disqualified from the hole since BGÇÖs infringement assisted A GÇô Rule 30-3f. If the balls do not collide, A is not penalized.