As others mentioned, it would require jailbreaking and yeah depending on the state could be big legal problems. That said there are a few products out there such as Mobile Monitor. Since your friend is also the account holder he could look on the phone bill to see what phone numbers have been texted (unless his wife is using iMessage which wouldn't show up). If your friend jailbreaks the phone and installs the software anything he really finds out couldn't be used at all (because of likely state laws). It would be for his own benefit.
A far easier way to do this is via the backup file. If his wife manually syncs and backs up her iphone via itunes then it is very easy. Just need one of several programs like iphonebackupextractor .com. It will export all the text messages into an excel readable format (.csv).
Usually though when it comes down to suspecting a spouse or gf/bf is up to no good they must likely are. If his wife is really having an affair there should be a million other ways to bust her without the phone snooping. As the other person here said, a PI could handle all that.
