A person's brain chemistry reflects it, when s/he "falls in love", or becomes infatuated with another person. I mean, there are physical, medically measurable things about it.
And it's quite a high. It works on the same parts of your brain that many illegal drugs do -- and some legal ones. It feels addictive, and maybe for some people it is addictive, or it becomes that way.
It's normal, natural, and human. God made the process a part of us all, in varying degrees, so that we'd perpetuate the race, at least. What makes it a problem is how we handle it. What should happen is a beautiful cycle of attraction, then comraderie, then attraction, then warm friendship, then attraction -- being attracted to our own spouse, falling in love with our spouse, all the rest of our lives together.
Your hubby could drop that lady, stop talking to her, and the desire for his next "fix" would fade away, just as with quitting a drug (or cigarettes or whatever becomes your beloved crutch).
He needs to want to do that, though. It doesn't help if you become his policeman, cutting off his contact. He needs to do it himself. And at least at this point it doesn't look like he will.
There's no one pointed single answer for you. Everyone has brought up good points. Pray, of course; and do not believe yourself to be second-choice material. Do nothing hasty about leaving him or making him leave -- although it could possibly come to that, if he's ever going to understand what he's doing to your marriage.
I'm serious about that brain-chemistry thing. Similar processes happen, with different hormones and other chemicals in our brains, when we as humans take care of our pets, or small children; when we are attracted to people we actually know and can be with, or to people we have no contact with at all, like the Facebook lady. Similar processes go on when we use various drugs, or when we look at pornography, or when we are being with and loving on our own spouse.
The process, the chemicals produced in our own bodies, is an innocent, neutral thing. God gave these bodies to us. It's what we do with the process that can lead us into sin and/or into hurting our loved ones.
Yes, the "spell" can be broken, because nothing is impossible with God. But, if He does not decide to intervene, it will still wear off eventually, if something allows a break in the brain-chemical-high cycle. Like, if Hubby chooses to stop communicating with the lady. Or, if she dumps him. Or, if he actually tries to go to her and be with her, and he finds out that she has aged and changed as much as he has, and they cannot ever have that fairy-tale thing he remembers from high school.
You know, he likely didn't even have "it" back then -- he's remembering a time of little responsibility and good health and youth; he's remembering his "lost love" through rose-colored glasses.