My neighbor may have a drain field problem. Problem she does not have access
To the back yard, it has a chain link fence with no gate. If she adds a gate
What width should it be, to get small trench digger through way to the back yard?

She should talk to the contractor that will be doing the sewer work. Taking down and then putting a chain link fence back up is not a problem. Chances are that more equipment than a small trencher will be needed.

Just disconnect the fence at a post end, unhook the top wires, roll the fenceand remove a section of top rail. After you are fnished, reinstall the fenbcing. If you need a gate, you will need to measure the track width of the track digger. Gates are usually 3', 4', 5' and up. Easier to take the fence down.

Thanks a million

Great answer