'Mate' is an interesting word to apply to another person, but Hermione takes it in stride. One of the consequences of living in Darrow has always been coming across the occasional difference, sometimes subtle, sometimes more marked. It's possible that 'mate' is a word that's commonly used in Elena's world. It's possible that it's indicative of some culture, or some species, that Hermione hasn't been made aware of. Whatever it is, it doesn't change the fact that Elena is a cherished friend of Hermione's.

"That's wonderful," she says, her voice full of appreciation and emotion alike. "I'm so glad that you have your person here in Darrow. I'd love to meet him at some point, if he's amenable — but I understand that wedding receptions can be tiring for some."