I would mail the returns.  There are too many risks involved with giving the data to somebody else (and  you probably need written authorization from each client to do so).

In direct answer to your question, the other person would need to have ProSeries, register their EFIN with Intuit and you would give the data files to the other person to e-file.  The data files are usually in the C drive, the BasWin19, then 19Data.

But again, giving the data to somebody else is risky and probably requires the clients' consent.