Are there apps where friends' pets can visit each other?

Yes. In Migachi you add a friend with a friend code, then visit your friend's pet and play together. You can also share a goal with a friend and they can cheer it, and every goal starts private. There are no nudges, so nobody is pushed to do anything. Each pet lives in its own little pixel house, and a visit is a small, friendly moment. Migachi is an iPhone app, coming soon to the App Store, for adults 18 and older. You pick dog or cat, name your pet, choose a few small daily goals and earn coins for the Shop. Friends are a gentle extra, and the app works just as well on your own. It is a subscription, with details shown on the App Store before you pay.