Even if it is not a common way, it seems that we can use an infinitive clause with no object for a purpose statement.
We talked here before about the accuracy of the question
Who did you find to study with?
and everyone agreed with the idea that "to study with" is an adverbial relative clause there stating a purpose. Then we can see that infinitive clauses don't have to be attached to nouns.
The other example;
She is hard to study with, because, she is really lazy.
Again, "to study with" doesn't need any noun for its own preposition "with".
Let's come to the main point now.
These details make the following sentences grammatically/linguistically correct.
To study with, I found someone who is one of the most successful students at the university.(Why did you find him? To study with.)
According to some people, we have to say:
To study with him, I found someone who is one of the most successful students at the university.
But they don't have any evidence for it.
We don't say:
"She is hard to study with her." or "Who did you find to study with him?"
I believe that in those kinds contexts, we don't need to use a pronoun because using it is more confusing..
What do you think?
If you think that we have to use a pronoun, what is your evidence/reason?