Idiomatic way to say "doing something for somebody"

Question

What is the idiomatic way to say "doing something for somebody" when they are supposed to do it, not you ?

Say for example a school mate asks you to do the assignment she's been given.

How would you say it? "He asked me to do his assignment [don't know how to say it in place of him?]"

UPDATE:

Following some of the comments I received, I want to clarify that the context is not necessarily academic cheating. What I'm trying to find -provided that it exists- is a translation from my language (Italian) of a subtlety in meaning when you do something for somebody who were supposed to do this thing and you decide, whatever the reason, to come forward and carry it out for her.

A couple of examples more:

• a colleague is assigned a task, but can't (because she doesn't know how) or doesn't want to do it (maybe because of laziness). Then you do the task for her (either because she asked you for help or you simply decided that somebody had to do it)
• a death sentence is handed down to someone and you step up begging to be taken instead

How would you rephrase the [for somebody] part in the two sentences above? Again, maybe I'm struggling because I can see a slight difference in meaning in Italian, when perhaps in English there is no straight translation

Answer 1

There are a few phrases that you can use, such as in place (of), instead (of) (also stead), to express the "doing something for somebody", depending on the context.

In any case, the phrase for someone doesn't have to mean that the person you do something for must be lazy. This, again, depends on the context. Compare:

He asked me to do his assignment for him.
I offered myself to do his assignment for him.

Here are some examples of in place (of), in someone's place, instead (of), and stead, I found on the web:

John came to help in place of Max, who was sick.
When the king's cloaked challenge is answered, Fluellen stands in place of him, representing him, and receiving the blow.
She knew she should not have sent him to work in her place tonight.
"O king, take my life instead of hers. Let me die in her place," he pleaded.
... it was Judas who was crucified instead of him, ...
... she had to ask their family doctor to prescribe instead of him making it available to begin with.
Take me instead! Not my boy!
The chairman spoke in her stead.
The marketing manager was ill and her deputy ran the meeting in her stead.

So, you can say any of these followings (choose wisely, register is important),

He asked me to do his assignment for him.
He asked me to do his assignment in place of him.
He asked me to do his assignment in his place.
He asked me to do his assignment instead.
He asked me to do his assignment instead of him.
He asked me to do his assignment instead of him doing it himself.
He asked me to do his assignment in his stead. (formal)

Answer 2

As far as an idiomatic way of phrasing it, some phrases I would suggest are "picked up the slack" or "took up the mantle", e.g.:

• "Karen didn't know how to do the task she was assigned, so I picked up the slack and completed it on her behalf." ("Pick up the slack" implies that the other person is not doing their fair share and you are compensating for them, either as a favor or out of necessity because someone has to do it. "Slack" is like loose rope drooping down when you're playing tug-of-war or something and is an indication that someone is not contributing to the best of their ability.)
• "After Cathy quit the club, no one wanted to take over organizing the bake sale for charity until Bob took up the mantle and saved the day." (This idiom comes from the Bible. In the Bible, a prophet named Elijah left his cloak behind when he ascended to heaven. A new prophet named Elisha picked up this mantle to wear and also took over Elijah’s role as prophet.)

Answer 3

It's like doing something on behalf of someone else. So you will do it on behalf of that person.

Answer 4

Another term for this concept is "to do something on someone's behalf"

John was to busy to complete project, so I did it on his behalf.

This phrase is also, perhaps most often, used when one person or entity is acting as an agent for another.

I asked the bank to send the payment on my behalf.

In general, "on X's behalf" means "in place of X, and for X's benefit" or "as a substitute for X". It does not suggest cheating, but a legitimate or requested substitution.