Translation:The guy cooked for his new girlfriend.
You probably mean för, but that doesn't work. för is most typically used to denote an audience of some kind. Jag visade boken för henne 'I showed her the book', Jag berättade för honom 'I told him'.
When you do something for someone, you can use till – the normal preposition for recipients, or åt – often used to denote 'doing something so that someone else doesn't have to' (but it can also mean more or less the same as till, this varies a bit with different verbs).