Is there a word/phrase for when you ask for assistance/advice and they just do the entire task for you?

In British idiom we often use take over in this context:

"I really wish he would help instead of taking over"

to take over = to replace someone or something

Cambridge

take over = phrasal verb ; If you take over a job or role or if you take over, you become responsible for the job after someone else has stopped doing it.

His widow has taken over the running of his empire, including six London theatres.

He took over from his uncle as governing mayor.

She took over as chief executive of the trust.

Collins

and a final example chosen at random:

Sometimes she helped Henrietta, the cook, whom everyone called Miss Hen, and on weekends Thrower often took over the cooking. She took over the cooking fulltime when Miss Hen died. And she stayed on for more than 50 years, until the Hollanders died, then worked for one of their children.

Washington Post

To give a negative feel, you might in some contexts use arrogate, as in "He arrogated the task to himself" but the word is just not used enough nor understood widely enough to qualify well. usurp is another possibility but with more limited meaning.

arrogate = to take something without having the right to do so:

"They arrogate to themselves the power to punish people"

Cambridge

usurp = to take power or control of something by force or without the right to do so

"He frequently usurped the powers of the commander, to whom he felt superior, especially if the latter was a non-party person"

Cambridge