Is there an English word or phrase that means "intending to help and hurting instead?"
Solution 1:
Counterproductive. The TA is being counterproductive.
His uncontrollable anger is very counterproductive to his attempt at saving his marriage.
http://www.merriam-webster.com/dictionary/counterproductive
Solution 2:
The TA is doing/causing more harm than good.
to be damaging rather than helpful
Usage notes: usually said about things that are intended to be helpful but do not have a good result
[The Free Dictionary]