There a more decent ADA seats at TCF than most stadiums I have been to. There are tons of them on the upper level that are empty much of the time running all around the upper concourse from the end of the club/press section all the way around to the big scoreboard.
Those upper seats can be accessed via three elevators,1 #1. behind the student section, #2 gate C/ 50 yard line on "sunny side", and 3. the open end, inside the big arch, to the left of the main stairs, near the scoreboard.
There are also some ADA type seats in 141/142 right behind the Gopher bench that probably hae a ramp to them from the hallway near the historical display/ hall of fame area. Those are very prime seats and there are not too many of them.
I think people tend to squat in the large number of HC spots in the main concourse ringing the field. People tend to lurk and if no one is there some will sit down. The ushers do keep an eye on those spots and tend to intervene if they know the regular occupants tend to be present, although the U of M may not offer those for single game seats.
If the person you know is capable of handling a step or two, I'd sit take the elevator up from gate C and sit at the top in one of the empty ADA seats in the 20-50 yard line range 209, 210, 211, and in the worst case you can move down a step or two the empty seats at the top of the section. I have rarely not seen empty rows up there, and the last resort are the benches nearer the scoreboard where the often absent visiting team fans are.
I sat in a really nice ADA compliant seat this year at the Nebraska game, on the east side on the walkway at the bottom of that new upper deck visible on TV, right at the 50 yard line. I was talking to a woman in her 50s who was with her husband, who was disabled and could not really speak, and she told me no one ever sits in the folding chair next to their two spots, so I sat down for the first half. These were surprisingly great seats at the 40 yard line, considering how high up that area looked on TV previously. The environment in Lincoln, sitting in the sun on that side was very impressive.
I did notice that a great deal of outwardly able bodied people seemed to be sitting in most of those seats, but maybe they have a surplus of ADA seating there.