How can a Green Card holder prove their length of absence when exiting and entering through a Mexican land crossing?

Me and a friend of mine who is a Green Card holder are planning a road trip from Seattle to Tijuana by car. To avoid a long queue we are planning to cross the Mexican border on foot and go back after a few hours, hence no FMM would be required from either of us and neither would Mexican immigration stamp our passports.

Here comes the catch - as a Green Card holder, my friend cannot stay out of the US for more than a few months at time. But given that the US land border doesn't have exit immigration and neither does Mexico have entry immigration at Tijuana, there would be absolutely no record of our absence from the country. And therefore my friend won't be able to prove how long he's been out of the country when crossing back to the US through the land bridge.

What documents does the US recommend having in this scenario to prove you haven't actually stayed abroad for a few years? Or is my friend unnecessarily concerned and no one would care about his absence at the land border crossing?

Note that this issue doesn't concern me as a regular visa holder as my I-94 would still be valid at re-entry and the US doesn't care about anything else at this point.