Why isn't my mini fridge getting cold?
"...a brand new mini refrigerator in good condition except...". Such a deal. The cooling system in these low cost mini fridges is the main part of the things. It is almost a sure bet that any replacement parts to repair the compressor or evaporator will cost as much or more than a new unit at a high volume retail outlet. I would recommend not even spending the time trying to troubleshoot it. Maybe you may do good in talking your neighbor into taking the unit back so you do not end up with the hassle of disposing of the thing.