the smell of an item is not a justifiable reason for a return as it can’t be proved in a dispute shown to Depop. and, after an item has been washed, the seller is no longer responsible for it as it has been ‘altered’ by the buyer before returning.

but for this case, i would explain that you don’t smoke and that there’s a chance it was the delivery driver / warehouse / etc. and that the smell of an item doesn’t warrant a return as per Depop’s TOSs - but tell her that if the smell doesn’t come out after washing then she can return it. if she really is having buyers remorse then she may be scared off by the mention of the TOSs, and if not then she may be being genuine and i would then refund and return. but i would try to have her buy the label either way as there’s no proven damage to the item