This answer is part of Pet's Spring Cleaning Campaign. This question is old, but this answer will still help people with the same problem.
Do dogs need dog friends?
If you (your dog's owner) are often away, such as being at a full-time job, your dog will need something to keep them happy and not lonely.
Yes, dogs do get lonely. Dogs are descendants of wolves, pack animals who do pretty much everything together. Wolves sleep together, hunt together, eat together—you get the picture. Though dogs have a long history of domestication that separates them from their wild ancestors, they’re still inherently social animals.
Most dogs have also inherited a drive to work, usually alongside their owner. Whether they were bred for hunting, herding, or protecting, dogs enjoy mental stimulation and social interaction. Being left alone for long hours without a job to keep them engaged can take a toll on their well-being.
There are lots of ways to keep a dog happy while alone. This can include toys or other distractions. Another dog is a great way to solve separation issues (as long as they get along)!
However, it is not the only way, and yes, you don't need a second dog to solve separation issues (training and other distractions should also be used).
So your friend was right in that a second dog is not absolutely necessary, but wrong in that there wasn't a reason to keep them "glued at the hip". A playmate is perfect for dogs who are alone. Again, dogs are social animals and need constant play, exercise, attention, and love!
Recently I had the opportunity to send the Maltese off to live with my elderly father, who lives in a different town.
Separating your dogs is risky. Once they know each other, they form a special bond. Sending one away is cruel to both dogs. Your dogs will get stressed and be unhappy.
petwellbeing.com supports this:
If you go through with separating your pets, understand that this change may be very difficult for your pet to process. They may start showing signs of stress or separation anxiety, including lethargy, whining, barking or meowing excessively, clinginess, hiding or even aggression. Other behavior may also change.