Depends on how they're restrained.
Since you've clarified that the creature is trapped in the coils of a giant snake, no, they won't get released. Repelling Blast throws the snake backward; this won't untangle whatever is wrapped up in it.
But the Grappled condition says...
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.
Yes, I know. Here's the thing: All that rule says is a creature can't continue grappling another creature that's out of reach. It's still up to the DM's judgment whether the snake in fact moves out of reach of its victim. It's perfectly consistent with the rules for the snake and whoever it's grappling to get pushed as a unit.
Repelling Blast says that it pushes its target 10 feet back. This is not absolute. If the target is standing in front of a stone wall, I don't know a DM anywhere who would rule that they're pushed right through the wall, even though the invocation doesn't say "10 feet, or until it hits something solid".
Now replace that stone wall with another creature. Does the target stop moving when they hit a stone golem? An elephant? A dog? A stirge? At some point the answer is going to change from "probably yes" to "probably no, and the other creature either gets pushed with it, or moves out of the way".
Now, what happens when it can't move out of the way? Suppose the target is roped to another guy. There has to be a point of failure somewhere: either the target doesn't move, or the other guy moves with it, or the rope breaks. Repelling Blast doesn't say it moves other creatures, but it also doesn't say it breaks ropes, so the DM will need to rule on where the system stops behaving ideally.
In this case, the snake is the rope, and snapping it in half so that the constricted creature can escape is a pretty drastic (and abusable!) outcome for a 10-foot knockback. More reasonably, a DM would decide that either the snake doesn't move, or it brings its victim with it.