If he isn't already using electrum he could export all the keys from his old wallets, then import them into electrum. After that he could freeze any problematic dust addresses, so they can't be spent from.
Either that, or wait until the fees are extremely low, then send dust spread across loads of addresses to a single consolidation address.
Fees are fairly low at the moment.
Today the bitcoinfees website says zero fee transactions can still confirm if you are prepared to wait 95 blocks.
It says a one sat per byte fee can confirm inbetween 3 to 64 blocks.
https://bitcoinfees.earn.com/
*edit*
Apparently it's very difficult to broadcast a zero fee transaction. There seems to be a way to trick electrum into letting you broadcast a zero fee segwit transaction, but I'm not sure if it works for normal transactions.
https://www.reddit.com/r/Electrum/comments/kt41dq/finally_changed_electrum_code_to_have_truly_zero/
OP_RETURN is a script opcode used to mark a transaction output as invalid. Since any outputs with OP_RETURN are provably unspendable, OP_RETURN outputs can be used to burn bitcoins.