First, I think しないでほしい is usually not used with ～に in this situation though ”してほしくない” is often used with ～に.
In my opinion, しないでほしい is usually used solely. The object is "you", and it is usually omitted.
そこに座らないでほしい。 I want you not to sit there.
Aに...しないでほしい can be used with "と言った", "と思った" and so on in a sentence like this:
私は彼にもうこれ以上罪を侵さないでほしいと言った。
I said to him that I wanted him not to commit sin anymore.
or
I said to him "I want you not to commit sin anymore.".
Perhaps, Aに...しないでほしい you learned may be as below.
彼に迷惑行為をしないでほしい。 I want you not to do him anything he doesn't like.
彼女に近づくことはしないでしい。/彼女に近づかないでほしい。 I want you not to be close to her.
In this case, there are two object unlike the above two cases. One is "you" and the other is "someone/something".
It is the case we want "you" not to do something to "someone/something".
If you said
そもそも合気道の達人に、一般人と8時間も組み手をしないでほしい。
I think this means...
In the first place, I want you not to do Kumite against 達人 (cooperating) with ordinary people for 8 hours.
However, In my opinion, the correct meaning is
In the first place, I want you not to do anything like that 達人 (like you) does sparring/Kumite with ordinary people (like me) for 8 hours.”
そもそも合気道の達人が一般人と8時間も組み手をするなんてこと、しないでほしい。
I think 遊 talks to 先輩 directly in his brain though she is not there.
By the way, there is another explanation like this or other answers.
Perhaps, I could be wrong, but I strongly feel it is weird to say/hear "AはBに～しないでほしい"... though it may be grammatically correct.
So, please note this is just my opinion as a Japanese.