"What benefits could he give her" could refer to her job at the massage place.
I would stalk her a little and find out more. Read her iChat when you can. follow her sometimes on the sly.
She is taking your car without permission, she knows its wrong, right?
That arab probably works there, thinks she's attractive, and tried to hire her. He probably knows that Chinese women like money, and promised her lots of money. Is she naive enough to fall for it? It seems so.
Tell you you don't want her to work in the Chinese massage, because they are shady places where lots of things can happen. If she objects, tell her if she does it anyway, then you two are done.
There are certain things that as a married couple, if the one spouse objects, the other cannot just go and do it anyway. Working in shady jobs like massage parlors is one of those things.
You should find out. Maybe hire a private investigator. If he finds something, it will be evidence you can use to divorce her properly, and not be held responsible for her afterwards. If not, then you can rest easy knowing she's a good woman. (but naive...!)
***
Let me tell you my own story about infidelity.
I was dating a girl when I was young. We lived in different cities, a half hour's drive apart. One night, I was going to go to see her. But I was really tired from work, so I called it off.
When I got home, I felt better, so I decided to surprise her, by showing up at her work and meeting her when she got out. I got there a bit early, so I parked in the other side of the lot.
When she came out, another guy met her, and they were very happy to see each other.
I decided to wait and see what happened. They each got in their cars, she drove home, he followed her. (and I followed him, spy-style, a few cars back) They went back to her house. It was late, they got out and started to go in together. Then I pulled up, and said: "WTF! Who the hell is THIS!?" She told me it was an old high school buddy. I asked why the hell is he coming home with you late at night? She said it is not what it looks like. I said "BS. Go ahead and screw him. We are done."
My point is that you can find out, and know for sure. NOT knowing is the worst.
Worst case? She will catch you at it, and play the old: "You don't trust me?!" card. Then, you can say she is acting suspiciously (give the examples) and say you wanted to check.
Good luck man.