I'm going to do someone else's work, and let them pass it off as their own.
Why would let someone pass your work off as their own?
They seem to be struggling with their assignment and I want to help them out.
While it's not necessarily bad to help someone with their work, you shouldn't let them pass it off as theirs. That's pretty dishonest.
What's the harm in helping them? I don't really see it as dishonest.
If they turn in an assignment that they didn't do, it's dishonest. You should teach them how to do the work and let them do it on their own. That will be more useful for that person in the long run.