Feel guilty? Brian would never want him to feel guilty.
Is this a post 513 question because I don't understand? Justin isn't "leaving" Brian. They're agreeing to a long distance relationship but they're not breaking up. Eventually they'll live in the same city together again and they'd both probably have to travel for their work. To me, that's not leaving.
If they don't move together to a place in one city, and instead continue a long distance relationship, this question makes no sense to me either, because they're still together.
Also, what's to prevent Brian from feeling guilty for not following his partner for his career if we're discussing the abstract?