Is it permissible to accept a gift of earphones taken by a friend from an airplane, given that it is unknown whether or not this is allowed?

If you know that the headphones were taken from an airplane and that your friend's ownership of them is questionable, then do not accept the gift. This is because the original status of the headphones is that they belong to the airline company. A gift is not valid unless ownership is established, and the ownership of the headphones here is questionable. You should advise your friend not to take anything before ensuring that it is permissible to take it, and he must return the headphones if taking them was not allowed. It is not permissible to infringe upon the rights of others, no matter how minor.