Some of our friends from Russia will be visiting us in US on a visitors visa. We wanted to go with them to Cancun, Mexico. Will they have any trouble entering Mexico or returning to the US?
Mexico Entry: Mexico allows entry to any unexpired US visa holders for business or pleasure.
If they present themselves before the CBP for readmission within visa validity period they should be allowed back using their unexpired visas.
If their visa expires either for duration or number of entries while they are in Mexico, I would suggest getting Mexican e-visa (assuming they are Russian Citizens). Upon re-entry, within I-94 validity period (most likely, same as admitted until on a stamp) then they should be readmitted under Automatic Visa Revalidation if their visit is limited to 30 days, and they do not visit any other country in between.
I would suggest having an electronic I-94 printout, as well as some proof that they are going to leave the US soon (e.g. ongoing ticket). If their visa expires while they are in Mexico, it may take some convincing the airline representatives to let you board the plane back, so arrive early, have CBP automatic Visa revalidation printout on you and ask for English speaking supervisor in case of any trouble.