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davidcyclebackFor many old team signed ball, a batboy might sign a few of the names as those players didn't want to sit and sign 100 balls. This is fairly common. In other words, an old genuine team sign baseball can have many signatures signed by the real players and a few signed by the batboy. The batboy signatures are called clubhouse signatures. An authenic such ball can get a LOA from reputable autograph expert, with the LOA likely listing the clubhouse signaturure in the letter. Even with a non-Mantle, if the ball is authentic it's worth a lot more than was paid.
Many old team signed baseballs were shellacqued. The shellacque will lower the value but a shellacqued ball is still collectable and worth something. How shelacque or other substance effects the value of an authentic autographed item largely depends on the condition ('how it looks.').
As I mentioned once before, if you or your friend want to do a quick and dirty look into authenticity on your own, compare the common and minor players signatures to known real examples. A forger would focus on the Maris and Mantles of the team, not the minor players.
Yankees stuff in general is very popular and anything involving Mantle and Maris even more so. If the items are authentic and you can get an LOA from reputable and well known autograph expert, they are worth good money. If you had a ball signed by 10 1961 Yankee common players (no stars or even semi-stars)you could probably resell the ball for more than the $100.