This address belongs to a wallet that contains 137 addresses that were (at one point in time) funded.
https://www.walletexplorer.com/wallet/02101970cc8b344e/addressesThere were points in time the balance of the wallet was close to ~5 BTC
https://www.walletexplorer.com/wallet/02101970cc8b344e?page=2So, i think it's safe to conclude that your customer is either somebody that's been around for a long time, and did tons of transactions (in which case he'll have to figure out what went wrong by himself) OR your customer is using a non-custodial wallet (web wallet, exchange wallet,...), in which case he'll need to contact his non-custodial wallet's support team.
My gut feeling is that it's the second option: a newbie that's using a non-custodial wallet but has no idear how things work. He created an address, gave it to you, you created a transaction funding this address, but his non-custodial wallet isn't updating it's records (either because they have technical problems, because they're a scam, because they're monitoring the wrong chain, because your customer copied a donation address instead of an address belonging to his account, because the customer's pc was infected with a clipboard virus,...).
Anyways, it's not up to you to solve his problems, however, you might want to consider giving him some best-effort support. Maybe let your support engineer spend one or two hours trying to find out which wallet your customer used help him find out if they scammed him. Do make it perfectly clear that you're not responsible for their choice of wallets, and there is no way you can actually recover the funds for them, nor will you reimburse them if they were scammed by the wallet provider they chose.
If the address hadn't belonged to such a big wallet, i'd say dothebeats' post was also worth looking into... You wouldn't believe how many times a newbie creates a BCH wallet and funds an address from this wallet on the BTC chain (or any other fork combination).