This is the harsh truth of the corporate world. Someone who has come into the organization will have to leave one day.
Friendships at work are weird things altogether. You see your colleagues at least 5 days a week, 24/7, have meetings with them, go to picnic sometimes, discuss your work, personal life and many more things.
Each Friday, you run your work friends with your weekend itinerary, and then you both go through the Monday Blues when everyone starts late and talks about how fast the weekend went.
You and your colleagues (close ones) would have lunch together, sit in long and boring management meetings together, chat at work events, make coffee or tea for one another, and even support one another through stressful periods. You will slowly start getting insights into their personal life, their work aspirations, and their holiday plans too.
Usually, they are the ones that know you better in the stressful environment and they may share the same pressures on a different level with you.
One day, they just walk by and tell you that they are about to leave, as they have found a better opportunity, and at that same point you feel some void in over your stomach as if someone has punched you on your tummy, and the only thing that comes out of your mouth is "huh!!!"
No more talking about your favorite TV shows, no more work best friends. How on earth are you supposed to move forward from something like this?
As they say, absence makes the heart grow fonder.
Conclusion:
Your friend leaving your organization doesn't mean they are not there to talk to share their wisdom with you. You can always connect with them on Social Networking Platforms like thewiki Network.
You can also send them emails to feel as if you are still in the same company but in different departments.
In a nutshell, moving on is the way of life and you should also try and do that when someone decides to leave. Make use of this time to enrich yourself with hobbies and upskill yourself for the next big opportunity in the organization.