Pkrs or Pokerus doesn't do anything harmful to Pokemon. The opposite of it, it makes your Pokemon gain the twice of normal EVs.
When a Pokémon has the Pokérus, it gains double the effort values from battling (e.g., fighting a Magikarp will give two Speed EVs, rather than one). Effort points gained from vitamins and wings are not doubled. The infected Pokémon can infect other Pokémon with the virus for a period of one to four days. However, the Pokérus timer can be delayed by several methods such as by placing the infected Pokémon in a PC Box.
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Since it lasts for only 1 to 4 days, your friend has told you to not withdraw it out of PC. If you kept the infected Pokemon in PC Box, the Pokerus timer will be like delayed.
Due to the beneficial nature of the Pokérus, players will often place an infected Pokémon in the PC where it will keep the infection indefinitely, so that it may be withdrawn to spread the virus at will.
After the Pokerus timer is finished, your infected Pokemon is cured and can't spread Pokerus to other Pokemon. However, the doubling of EVs still remains.
Once a Pokémon's immune system has fought off the virus, they cannot spread it further, nor can it be spread to them by other Pokémon. In Generations II and III, a dot will appear on the Pokémon's status screen to indicate that they have had the virus previously, while in Generation IV and onward, it will be a small face. Though the virus is gone, the Pokémon will still gain twice the EVs it would have gained before the virus.
That's about it. You don't need to worry about your Pokemon getting infected, you had tried the best to keep it in PC Box for the timer to be delayed.
About EVs or effort values
As Pokémon battle, in addition to gaining experience points they also obtain Effort Values (usually referred to as EVs), which are special hidden values in each stat (HP, Attack and so on). Each Pokémon you battle against gives off EVs in one or more stats.
Generally, this relates to the Pokémon's strongest stat - for example, if you battle a Geodude your Pokémon will get one EV in the Defense stat. If you fight a Gengar, you will get three Special Attack EVs. (See the pages to the left to see which Pokémon give off which EVs.)
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You should probably read this article— it will give an in-depth explanation of what are EVs.
Hope this helps!
