Because this ね is used as expectation for the agreement and replay like そろそろ出かけようね.
彼女とパーティに行けばいい means you should go to the party with her and this sentence is a suggestion, so this sentence added ね is unnatural because speaker doesn't expect the agreement and replay.
彼女とパーティに行けるといい means it is good that you can go to the party with her and this sentence added ね is natural because speaker expects the agreement and replay.