Disconnecting the console from the internet is the only sure way to prevent player invasions in Dark Souls 2.
That said, there are many conditions for both being able to invade and being susceptible to invasions, so your friend shouldn't have to worry too much. In my 72 hours of playing the game, I've been invaded only 3 times, not counting the areas belonging to the PvP-covenants. Those are entirely optional.
Like the comments above mention, you can minimize invasions while online for about an hour by burning a human effigy at a bonfire. The item description for the human effigy mentions this effect:
A warm, soft, shadow-like effigy. Use this item to reverse Hollowing.
It also weakens the links to other worlds, preventing invasions and
most cooperation. Peer closely at an effigy, and one begins to
perceive a human form, but whose form it takes depends on the person
looking. #
The Way of Blue supposedly summons available members of the Blue Sentinels covenant to help the Way of Blue player being invaded. I don't believe it matters whether the Way of Blue player is hollow or not, but the Blue Sentinel must be human, wearing the ring and also not fighting a boss or near a boss gate. With all these conditions, I'm not surprised people have seen few protectors these first few weeks after the game is out. Only a portion of players elect to join the Blue Sentinels, and then they both have to sacrifice a ring slot and not lose their humanity to be able to help.
Since your comment mentions the original Dark Souls, I'll add that in that game you would never be invaded if hollow; only in human form. So you could easily play through the whole game without disconnecting your console and never be invaded.
In my experience, invasions have resulted some of the most nerve-wrecking and adrenaline-filled battles in the Souls series. They've also been some of the most memorable, so I recommend not trying to prevent them.
Can I invade a world with a friend? If yes, how?
Short answer is no. You can only invade a world alone. BUT, if you're a member of the Covenant the Way of the Blue, it is possible for another member of this covenant to help you when you're being invaded.
Advantages of playing online with the possibility of invade and being invaded?
There's actually a lot of advantages to online play!
PvP adds a huge thrill to your game. Being invaded can be quite annoying, especially if you're on some really hard monsters and you get killed. But, killing an invader is very satisfying and invading people yourself is alot of fun, too!
Online play adds co-op. This can be very useful for hard bosses, or just to have some fun with your friend(s).
Markers will be placed all over the world. They're placed by other players, usually with hints, tips or sometimes you can see how they died so you won't make the same mistake.
If you're having trouble with invaders in a certain area, try burning a Human Effigy. This is only possible with online play enabled and it will temporarily make you immune for invasions.
Best Answer
While Dark Souls can be played with friends, keep it mind its co-op features are more intended as random help than full-on multiplayer. The biggest reason for that is that progress is not shared between the host and phantoms.
The host must be in human form to summon phantoms, and the summon range is based on your Soul Memory range.
The cat in Majula sells the Name Engraved Ring. When you equip it you choose a god and you're much more likely to match with people who chose the same god. It roughly doubles the summon range, but not the invasion range.
1- Use the small white soapstone. You get it from a chest above the Cardinal Tower bonfire in Forest of the Fallen Giants. The host can use small signs even if the zone's boss is dead. The summoned player will stay for about 10 minutes, but every kill shortens the timer. They can help with some weaker(?) bosses too if you're close enough.
2- Use the (regular) white soapstone. You get it from Pate after clearing the area next to him in the Forest of the Fallen Giants. You can only see "regular" signs when the zone's boss is still alive. A phantom summoned that way will stay until the boss is killed, or until you timeout (though you usually have enough time to clear the whole zone just fine).
I'm not aware of any official information about cooldowns and timeouts but unless your phantom dies immediately after summoning it, you should be able to resummon it just fine. For example, I summoned a friend through a small sign at the start of the Shrine of Amana and played until he fulfilled his duty, then went back a bit to a safe spot and could summon him right back. Same deal with the regular sign. I summoned a friend to kill the Skelleton Lords and by the time I got to the Harvest Valley I could summon him again.