１つは友達の分だよって英語でなんて言うの？

You'll may want to start your answer by saying "no" so the person immediately knows that you will not be eating it all by yourself. "Of course not" is a way to keep the conversation lighthearted and fun and then you can state that you will, in fact, be saving one for your friend.

When we use the phrase 'saving some' this means that we are keeping something aside instead of consuming it ourselves so that our friend can eat something too. Another example may be - "I am saving some french fries for John for when he gets back in a minute."

If you are sitting alone when the food arrives someone might think all the food is for you to explain that you will be sharing with your friend then you could say 'no, this one is for my friend' or 'I'm saving some for my friend'

If I had bought a lot of food at a store or restaurant and somebody asked if it was all for me, it would probably be all for me. :) But assuming I had gotten something to share, I would say something like "oh, it's not all for me.. one is for my friend." or "I got one for my friend too." I hope this helps! :)

If someone asked you "Are you going to eat all this yourself?" and you want to explain that some of the food is for your friend rather than yourself, you could say any of the following: "One is for my friend", "I'm saving some of the food for my friend, I wouldn't be able to finish it all!" or "It isn't all for me, I will be sharing with a friend".

"I'm leaving the rest for my friend" simply indicates the rest of the meal will be going to a friend of yours. "I'll be sharing the rest with a friend" also describes that you will share the rest of the meal with a friend. "The rest is for a friend" indicates the same as above. All three examples are straightforward, commonly used and clear ways of describing what you will do with the rest of your meal, that is, share it with or give the rest to your friend.