Unless the SO had some reason to believe that his mate was cheating on him, he and we should just chalk it up to dreaming. We dream about all sorts of weird things, including people we know, or knew, or don’t even know at all.
No one knows if she was dreaming about another guy, but even if she was, unless she is interested in hooking up with that other guy, or is cheating on her boyfriend, it really doesn’t mean much of anything.
Also, most dreams are not very realistic, in the sense that they are not images of things that are actually happening, they’re all over the map, visually and emotionally. I don’t believe that dreams have inherent meaning. The man who’s name she called out may be someone who was chasing her, or an old friend, or a previous boyfriend (who she’s over) or somebody she barely knows, or someone she doesn’t know at all, or someone she saw on TV.