Some people have an infatuation with themselves being evil and in turn try to shift their own perceptions of their character into their dungeons and dragons character to express themselves. It's not something that should be ignored as an evil character in a group with good people would cause a problem, unless there is a reason for the alliance. And in order to understand why someone with an evil alignment would ally with someone with a good alignment, you must first understand evil.
Probably the single alignment most likely to ally with a group of good aligned player characters. A Lawful evil person is ever the manipulator, they use their own convoluted system of ideals and morals to advance their schemes. A Lawful evil character will most definitely hide their alignment and put up a facade. And will not commit evil acts willingly unless they know that they can pull it off without being caught, and never if it conflicts with a pact that they've created or a promise or deal that they have made.
Probably the least likely type of evil that you should realistically allow into a game. A Chaotic Evil character does what they want, when they want, and how they want. They will take things from people just to take them, and break things they want just to watch them burn. This will most likely get him killed by your good aligned group in relatively short fashion.
They aren't very courteous, will backstab people given the first opportunity, and will make a pact only as long as they know they obtain the dominant bargaining position, assassinating or killing someone when the other party gains the upper hand.
Probably likely to be a character in a game, albeit one that could still be good aligned, mind you. A neutral evil character will do things that will advance their own goals, such as attaining more money ( Neutral evil characters are very motivated by greed ) and will use the resources around them ( such as good aligned PCs ) to get what they want, be it women, money, power, or anything of the sort. Neutral evil doesn't mean dumb though, and if they're aware that the people in the party are "goody two shoes" they probably aren't going to reveal their hand to them.
So as far as allowing an evil character into a good aligned party, make that person very aware of the fact that he is joining a party in which if he turned against the party at an opportune moment that it would likely result in his death. If this happens, the person will likely be pissed, but you did warn him. Give him the opportunity to reroll if this happens.
Also, Make sure you know which of the evil alignments he's going to play, and make sure that you're clear in exactly how he's going to play his character to avoid the probing eyes of a groups paladin or if he's going to hide his alignment so that he can advance his goals.
If you can come to some sort of agreement involving this, and insure that he isn't going to play the Chaotic evil "Watch the world burn and smile", I'd say go for it. Just because someone is evil doesn't mean they are going to outwardly appear so.