The claim for N95 masks is not that they are effective for 95% of people, it's that they filter out 95% of small particulate matter. The remaining 5% may or may not be enough to infect anyone, or 5% of those wearing, or 100% of those wearing.

The manner in which you would make a legal claim that the product is not as advertised is to take the mask and use the same certification process the manufacturer claims to be using to test efficacy.