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The change to remove the restriction on who can release funds was part of OpenZeppelin 2.0 in October 2018.
- Removed restriction on who can release funds in
PullPayments,PaymentSplitter,PostDeliveryCrowdsale,RefundableCrowdsale. (#1275)
In your interface you could provide the option to specify any participant address and pre-fill this with the current users account address. This allows participants to release their funds using any account.
If your participants are interacting with the smart contract directly, then you may need to provide a guide that talks your users through the process to avoid any issues.
An alternative, depending on the number of participants (as you would be paying the transaction fees), would be to call claimRefund on your participants behalf.
As an aside Mist has been sunset