They go into limp mode because something wrong has been detected by OBD system. Are you sure it's "limp mode" ? If it is, he should have gotten the
Power Reduced message on the dash readout.
How do you get it out of limp mode ? Well, you need to find and fix the issue that caused it. Start with getting the OBD system working.

Fwiw. Posting the year, model and engine size would help. There are some differences depending on what he has.