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。<br>
I said to him that I wanted him not to commit sin anymore.<br>
or<br>
I said to him "I want you not to commit sin anymore.".<br>
 <br>

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.<br>
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.<br>

In this case, there are two object unlike the above two cases. One is "you" and the other is "someone/something".<br>
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。

This means...

> In the first place, I want you not to do Kumite against 達人 (cooperating) with ordinary people for 8 hours.

However, This is way off. Because 先輩 is supposed to be 達人 in your case.<br>
So, the correct meaining 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.”<br>
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。

遊 talks to 先輩 directly though she is not there.<br>
 <br>

Having said that, Most people would understand the correct meaning from the context even if you used "に" though it is wrong...<br>


By the way, there is another explanation like [this][1].<br>
I am not a linguist, so 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"...

Perhaps, it could be grammatically correct, but I cannot stop feeling it is weird...

This is just my opinion as a Japanese who lives/exists now.<br>
So, please note my answer is my opinion as a just Japanese.


  [1]: https://japanese.stackexchange.com/questions/73128/can-a-verb-be-modified-too-many-times/73129#73129