From the quoted lines, I just don't see it as a deliberate decision to go after Henry. I'm not saying that if Henry seemed like he enjoyed Isabelle's company - that she wouldn't. I think she was doing what was expected of her as an intimate friend of Catherine's - acting like she's interested in Catherine's interests. That stops immediately, of course, because it's not focused on Isabelle and Isabelle can't not focus on herself for long - but others can take the statement and say that she's a good friend.