Consider the situation with three characters: Gary the Grappler, Casey the Caster, and Victor the Victim.

Gary has Victor grappled. Suddenly, Casey shows up to help Victor escape. He isn't strong enough to physically wrest Victor away from Gary and is running low on high level spell slots. Out of options, he casts thunderwave. Gary succeeds on his check, but Victor fails.

Since Victor failed, he must move 10 feet away from Casey. Suppose this is enough to put him out of Gary's reach. However, since Gary has Victor in a grapple and succeeded the check himself, can he prevent Victor from leaving his reach?

##Thunderwave##

[..] Each creature in a 15-foot cube originating from you must make a Constitution saving throw. On a failed save, a creature takes 2d8 thunder damage and is pushed 10 feet away from you. On a successful save, the creature takes half as much damage and isn’t pushed. [..]

PHB pg 290

##Grappled##

• A grappled creature’s speed becomes 0, and it can't benefit from any bonus to its speed.
• The condition ends if the grappler is incapacitated (see the condition).
• The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.

I use the thunderwave spell in this specific case because it's directly referenced in the Grappled condition. But, I can see two results and arguments in this case:

• The grapple is broken because Victor failed his Con check and was hurled 10 feet away from Casey, and away from Gary's reach.
• Victor only failed his Con check. He isn't hurled by the spell yet (so the grapple isn't broken yet). Gary can try and hold Victor in place by holding a contest against Casey's spellcasting. If he fails, Victor is hurled away, and the grapple is broken.

Which is correct?

The larger question that I'm alluding to with this example is, if the grappler is unmoved, but the grappled is moved, is the grapple necessarily broken?

Consider the situation with three characters: Gary the Grappler, Casey the Caster, and Victor the Victim.

Gary has Victor grappled. Suddenly, Casey shows up to help Victor escape. He isn't strong enough to physically wrest Victor away from Gary and is running low on high level spell slots. Out of options, he casts thunderwave. Gary succeeds on his check, but Victor fails.

Since Victor failed, he must move 10 feet away from Casey. Suppose this is enough to put him out of Gary's reach. However, since Gary has Victor in a grapple and succeeded the check himself, can he prevent Victor from leaving his reach?

Thunderwave

[..] Each creature in a 15-foot cube originating from you must make a Constitution saving throw. On a failed save, a creature takes 2d8 thunder damage and is pushed 10 feet away from you. On a successful save, the creature takes half as much damage and isn’t pushed. [..]

PHB pg 290

Grappled

• A grappled creature’s speed becomes 0, and it can't benefit from any bonus to its speed.
• The condition ends if the grappler is incapacitated (see the condition).
• The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.

I use the thunderwave spell in this specific case because it's directly referenced in the Grappled condition. But, I can see two results and arguments in this case:

• The grapple is broken because Victor failed his Con check and was hurled 10 feet away from Casey, and away from Gary's reach.
• Victor only failed his Con check. He isn't hurled by the spell yet (so the grapple isn't broken yet). Gary can try and hold Victor in place by holding a contest against Casey's spellcasting. If he fails, Victor is hurled away, and the grapple is broken.

Which is correct?

The larger question that I'm alluding to with this example is, if the grappler is unmoved, but the grappled is moved, is the grapple necessarily broken?