Is he in the business of renting business equipment for profit?  If so it goes on Sch C.

If not, rental of personal property goes on the Other Income line.

It depends.  Need more info to determine. What is client’s business? What equipment was rented and why?

If this is his own business equipment, and he rented it instead of using it, that still makes it part of his business and would be reported on his business return.

But as everyone points out, that's a pretty vague question.