teapot
A person doesn't have to be "all bad" to fall into devilish habits, and the King of Cups isn't necessarily above emotional manipulation; he's usually well aware of others' feelings and able to keep his own in check quite easily... So maybe a form of Devil-like control now emerging is his tendency to withhold the emotional connection that the 6 of Cups person has been eagerly trying to encourage/"extract"?
Another possibility (not knowing the card positions, etc.) is that he's sensing chains in her behaviour... If she's after much more demonstrative "feeling sharing" than he is, or seeking too much emotional attention for his liking, it might seem immaturely cloying (6C) and an effort to ensnare (Devil). The King may be very understanding in that case but still careful to maintain some distance. Just more ideas...
Oh I like that perspective on the king, devil and 6 cups. Ultimately the sitter will need to decide whether it fits her relationship, but it's certainly something I will pass on to her.
Interesting point re the devil and sensing chains in terms of her - I had a lightbulb moment re this today, and realised she is the type of person who has held back in the past, and I know that right now she is over-thinking her situation in her head rather than talking to her partner about it in much detail. Of course he would sense that and react to it. Thanks for these insights!