Stereo store lied about free installation?

My mother in-law bought three remote car starters last Christmas. The sellers agreed to install them for free since she was buying 3. Now they say they won't. When my brother in-law got his installed, it was a big mess and they still charged him $25 for some stupid remote fee. He's got problems and didn't challenge what they said.
My question is: is there anything I can say to scare them? It's not about 25 bucks to me, it's about a stupid local store promising an old lady something they don't want to honor. When I go in there today, I want to at least put up a good argument. The guys working there aren't too bright.
 
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