This sounds like a bad idea on so many levels.

Does the client have and EFIN? If not all these returns will need to be paper filed.

You would be added as a preparer and your vitals will then be in this other persons software, for them to use in the future. Do you trust this person to only use your info with your permission?

I would expect the time commitment to set up 100 rentals and their depreciation, PAL and any other carryovers to be quite significant. The process to convert from one user to another is a possibility with ProConnect, but you should call support to find out how that might work. 

I WOULD ONLY AGREE TO DO IT ON MY SOFTWARE. If the client wants a copy of the file for their software, I would give it to them AFTER the returns are finished. I don't think I would want to trust that the client with control would actually file what I prepared. They may want to improve on the product before they submit the return that has your name on it attesting to it's truth and acccuracy.