It's hard to take a dog who has never played with another dog to the dog park and expect them to jump in. You don't want to find yourself, or your dog, in an overwhelming situation. Take your puppy to play groups or have them meet your friend's dog early on when you can watch for aggressive behaviour, like growling or possessive behaviour with toys. Once or twice a week is the perfect amount of socialisation for a new puppy, so they can play and be tired for bedtime. The amount of time spent playing may vary, but at least 30 minutes with other dogs twice a week is great. This will help ensure that your dog has a great time in the dog park later on!