If they start within 30 feet then the PC can't get out of range
Unless they are a Rogue or Fighter or similar.
If the PC moves 30 feet and casts then they are still within the 60 foot range of counterspell.
If the PC uses dash to move 60 feet then they don't have an action left to cast a spell, unless they can dash as a bonus action (Rogue) or use Action Surge (fighter) or something similar.
Since the counterspeller doesn't need their action for anything, they can always use dash to get back within 30 feet of the PC, ready to use their Reaction.
Is this useful? Maybe
What this does is is tie up the NPC's actions. If they is continually running after the PC then they aren't attacking the rest of the party. In online games, this is called "kiting".
As Hellsaint pointed out in a comment, this also means the PC is doing nothing. PC actions are usually much more important than NPC actions, so kiting in D&D is often not worth it for players. As a DM, using an NPC to kite a player in this way is a great tactic. evil laugh
The NPCs should be using battlefield control
The PC can't run out of counterspell range if they are in a web or erupting earth area, or if they are slowed or held.