I think it would depend on how the person who had the dog for 2.5 years feels and what kind of personality the dog has. If the people were planning on giving the dog back the entire time and are okay with it (and the dog does okay) then it's fine....if they weren't planning on giving the dog back after more than a few months its different. Also some dogs would adjust to this just fine, some would not. We had a similar situation, my sister told my mom to take Sydney (schnauzer) until she had her twins because she was having a difficult pregnancy. So my mom took her with the intent to give her back after the babies were born, well they were born and my sister said she didn't want her back and told my mom to sell her (which of course she didn't). Then a year or so down the road wanted Sydney back and my parents wouldn't give her back and I don't blame them. Nebo loves me but also loves my parents I think he'd happily go live with them for a while then come back and happily live with me again. Whenever they dog-sit he's perfectly content being there. The girls love them too but they are happier with me I don't think they'd handle that very well; there's no way Skya would be able to live with anyone else but me. You can tell Killer is attached to us but he's such a happy "just give me attention" dog that I think he would adjust anywhere with some time. However I wouldn't dog-sit somebody's dog for more than a few months and be able to just give the dog back....I'd definitely be too attached.
Bookmarks