Proportionally. The partner should understand this, and also understand this is standard and fair, and also that future investments will dilute proportionally as well (unless you really want to hamstring your business).
That all said, you can of course choose to make the partner whole for this round, but that is not sustainable as the company evolves and the partner needs to understand that is (usually) going to be better to have a smaller part of a much bigger pie in the end, and that is why folks accept dilution.
To me, this is an essential concept to get straight on between the founders now, or this will not work out well in the long run.