friend A and B travel to mexico. friend A accidentally brings expired green card instead of current/valid green card. airline rejects friend A from returning to USA and recommends they go to embassy or consulate. friend B returns to USA and will be returning to mexico with friend A’s current green card the following day. plan is to return to USA together the next day. is there anything to worry about? they are worried most about friend B being asked why they returned to mexico after one day and are returning once again to the usa. should they just tell the truth or is transporting someone else’s green card not ok? thanks SB
There is nothing inherently wrong with this plan. However US Customs and Border Protection has a fair bit of leeway and discretion (to put it politely) and it's entirely possible that Friend B will be subjected to a substantial questioning or no questioning at all.
If questioned its typically best to be open and stick with the actual facts plus any documentation that can help supporting them.
The alternative would be to have friend B return to the US as planned and then simply overnight ship the Green Card to friend A in Mexico with a suitable courier. That's safer and probably cheaper too.
There are lots of people that cross the border every day, so it should not be anything suspicious if you do it too. It is perfectly legal to go or fly to Mexico to eat dinner and them come back two hours later.
Also, as others mentioned, mailing is perfectly fine. Replacement Green Cards come in the normal mail, as do passports, drivers licenses, etc. Of course there is the small risk of loss (with ugly consequences), but if you insure the mail the chance is negligible.