1. People are friendlier in the country than in the city.

2. People are more friendly in the country than in the city.
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In the dictionary, the comparative of 'friendly' is 'friendlier.' What about #2? Can we use 'more friendly'?

I would say yes, you can.

Kat is right. Both forms of the comparative are fine.

Yes, in English grammar, both "friendlier" and "more friendly" are acceptable forms of comparison for the adjective "friendly."