You need to have the correct total area of vent, regardless of what or where they cut. Code requires either 1 sf vent per 150 sf of floor area (1:150) or 1:300, depending on other construction considerations.
The amount of ridge vent required depends on the specific product they installed. Cutting only 1 side might impede the air flow to some extent since it has to move around the ridge board, but it's probably not a huge factor.
The worksheet for calculating what's needed can be found here:
https://www.certainteed.com/resources/802.pdf
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