I've noticed that I have far more conversations with strangers when I'm out walking my dog than when I'm out by myself. I've also made acquaintances with the regulars at the dog park. Pets aren't a replacement for human interaction, but they can help facilitate it.
Excuse me, this is bad advice. If someone is lonely they will have problem to find anyone who can take care of the pet when they can't (such as when they are away).
There are pet sitters and pet boarding services in most cities.
Which must be usually booked in advance.
Really, pet care can't be handwaved off and downvoted.
Lonely people can still plan their lives and act as adults. I'm one of them, a cat sitter is watching my cat right now. In any case, usually having your friends pet sit has to be discussed in advance as well. You have to hand over the house key etc.