@dwizum's answer to respond with ways to prevent the situation in future is great, but in addition you should also make sure that the developer's extra commitment and unpaid overtime is visible outside your department, and in their permanent record. When it comes time to look at pay-rises, bonuses or even who to keep and who to cut from a department, this is the sort of thing that should make a difference and there is no guarantee that you will still be their manager at that point.

Write a memo to your superiors and peers, praising the employee for what they did and make sure a copy goes in their file. Give a copy to the employee - having your thanks in writing makes them more tangible and gives them something specific to call on when discussing raises etc..

@dwizum's answer to respond with ways to prevent the situation in the future is great, but in addition you should also make sure that the developer's extra commitment and unpaid overtime is visible outside your department, and in their permanent record. When it comes time to look at pay-rises, bonuses or even who to keep and who to cut from a department, this is the sort of thing that should make a difference and there is no guarantee that you will still be their manager at that point.

Write a memo to your superiors and peers, praising the employee for what they did and make sure a copy goes in their file. Give a copy to the employee - having your thanks in writing makes them more tangible and gives them something specific to call on when discussing raises etc..