In your post, you don’t mention how your relationship with your wife is. Assuming it’s bad, then it’s time to file for a divorce and get on with your life.
If, in the rare event that your relationship with her is still relatively good (meaning: you talk regularly, do things together, have sex, et cetera), then I think the situation needs some more consideration than simply saying that you should leave her and get on with your life, however, that might be what you end up doing anyway.
First off, you shouldn’t be asking what she is to her wealthy client, because it’s not his feelings that really matter here. It’s hers, meaning, you need to find out what he is to her–does she care for him, does she even like him, or does she simply see him as a paycheck? She may even be seeing this as some sort of a way to “cut back” on escorting, because she’ll be having sex with less men but still making the same amount of money. There’s really no way to find out what he is to her without having a serious talk with her, which is something you should have had FAR before this point.
Next, you need to find out what YOU are to her. The reasons for this one should be obvious.
You should also find out what her reasons for continuing to be an escort are. Is it just the money (you say you make a decent living now, but I’m sure the “extra income” is still quite significant), or the thrill, some combination, or what?
Finally, you need to figure out what you can live with, and also what she can live with. Are you OK with her continuing to be an escort (from your post, it doesn’t seem if you’ve had much of an issue until she became “exclusive” with this client)? If so, for how long and under what conditions? Is she willing to stop at any point or for any reasons? And so on. It’s possible you two, depending on the situation, could work out some sort of compromise you could BOTH live with, such as her continuing to be an escort under certain conditions (assuming that’s acceptable to you), or her finding another job in a “similar” line of work (such as becoming a stripper, but one that’s NOT actually having sex with any of the clients). In the event you can’t find a compromise that works for both of you, get a divorce and get on with your life.