My Dixie had Jet, a lab x retreiver when she came how with us, they slept together and were unseperable. When we lost Jet, Dixie was 4 months ols. She got very depressed. We made sure to keep her happy playing games and going interesting walks.
Since then we have had a few fosters and dogs to stay, she isn't phased by it now, but to start with she was a bit miffed when the other dog left.
If you have any friends with dogs then go walks with them
x