I play this character. My GM and I discussed the build extensively before starting Storm King's Thunder because I was worried about this. Both from pre-planning and from experience, here're the things we've come up with:
0. Discuss this with your player.
Be open and tell the player that you're struggling to engage all of the party while allowing their character to shine when in its element. Ask them for the limitations they see to the character: any you invoke that came from their mouth will seem inherently "fairer" than those you invoke on your own. Some would call this metagaming, I call it being explicit about the social contract. "We all cleared our schedules, so let's try and make it fun for all to play."
That 600' range (I'm assuming a longbow in this part) can let a sharpshooter get off four or five "free" shots before a foe has a chance to respond. If they have a sightline. Yes, the sharpshooter ignores 1/2 and 3/4 cover. But they don't ignore total cover, which is what's granted by an interposed building, hill, &c. And at 600' off, moving laterally to "see around" a small hill is going to take your sharpshooter some time.
Real-play example: on the road my party encountered some giants wrecking a small keep. I got one shot in, and the giants circled to the other side of the building. While I was circling around to get a look at them the rest of the party snuck into their effective ranges, and the encounter basically started off with giants and rest-of-party at 100', me at standoff range. Only got one "free" shot.
That sharpshooter's great at delivering a big punch at a low-AC target. But 25 damage vs. 15 damage is moot to a 10hp target. I understand you can't play too much with encounters' composition (because of AL), but you can play a lot with monster tactics. If the encounter has a mix of big-target and small-target foes your sharpshooter would much rather be swinging for the fences against the big target. So rush them with little targets.
Real-play example: a giant was coming through the forest, but just as we started to see his head a group of I-forget-whats came out 100' ahead of us. I could take my shots at the giant, but a dozen magmin were bearing down on us....
3. Focus on party-mates.
The nice thing about being the sniper is that you stay relatively safe, raining death from a minute's sprint away. The not-nice thing about having a sniper is that the damage your opponents are dealing tends to be concentrated on \$N-1\$ party members rather than on \$N\$ party members. That sniper can't stabilize an ally or drag away a body. It doesn't take too many times choosing to stay safe before you'll see a mate drop and there be nothing to do about it. Choosing to stay at standoff range also chooses to write oneself out of a lot of interactions.
4. Size of strike.
So far these have all focused on the range-portion of Sharpshooter. But what about the size of that strike? I'd claim it's not actually that large, as compared to other characters built to deal single-target damage. For example:
A Sharpshooter (feat) archery (fighting style) Ftr4 can expect +3 to hit & 19.5 expected damage (d10 weapon damage, assuming Heavy Crossbow + 4 DEX + 10 sharpshooter).
A dueling (fighting style) Ftr1/Ro3 can expect +6 to hit & 17.5 expected damage (d8 weapon damage + 2 dueling + 4 STR + 7 sneak attack). (And we haven't even piled a feat onto there.)
A Dual Wielder (feat) two-weapon (fighting style) Ftr4 can expect +6 to hit & 17 expected damage (4.5 weapon damage + 4 DEX, twice).
Now the expected damage per attack between those two will depend on AC, but for many ACs the duelist fighter/rogue or two-weapon fighter is delivering a bigger punch, just as sustainably. In effect, with a sharpshooter you're trading a bit lower damage for safety (from range).
You mentioned how large the damage gets once your sharpshooter grabs second attack and piles on an action surge for a third, but that's. frankly, a red herring. Any fighter build is going to have that "problem"--it's a class feature, having nothing to do with Sharpshooter.
5. And sometimes, let them shine.
Sometimes you'll be fighting a crowd of kobolds in their warren of tunnels, and the sharpshooter'll effectively be mooted. And they'll bemoan the opportunity cost of not taking Sentinel or Polearm Master instead of Sharpshooter.
But every once in a while they'll be standing on a battlement on a sunny day with a clear view to the forest's edge a half-mile out and three giants approaching. With a smile, they'll turn to the party and say "don't worry, I got this. [Aside] 'I love it when a plan comes together.'"