If the person giving the massage was a woman, would you care? I bet not. It's just because that person is a man. Now, let's say he's a gay man. You probably wouldn't care about that either. It's just because you view your wife as an attractive female and the person giving her a massage as a competitor. But, your wife doesn't see him that way. That's the major thing here--your wife doesn't consider this man you're competition. If she doesn't even think of him that way, why should you? She's the one who would have to want to cheat on you with him, not you. Furthermore, because this guy IS a professional, he sees hot women for massages all the time. He probably sees hot, single women all the time, and therefore has no reason whatsoever to want your wife, no matter how wonderful she is, because she's unavailable on so many levels (his friend's uninterested, re-married ex-wife? Dude, did you miss all of high school and college, or just the sections where they taught dating rules? That's SO off-limits, it's not even funny!).
You need to trust your wife a little more. She's getting a neck/shoulder massage from a friend in a public place with you there to supervise. If she was meeting him for two hours a week in his posh, private parlor and wouldn't let you know what he massages or let you come see what's going on, then you'd have the right to be worried. But there's nothing more than a neck/shoulder massage going on here, and you and everyone else can see that's all that's happening. Nothing erotic, nothing intimate, just a little shoulder massage. If you're really worried about it, though, you could always ask said man to teach you how to massage the way he does, so you can do it for him when she wants a massage.