Since I don't know what Mac you're talking about, nor what version of the OS, I will speak generically.
What your friend did was probably formatted the disk as an NTFS disk. No, you won't be able to see it or access it.
You need to go to Disk Utility, which came with your operating system, and run that. When the utility scans your computer and turns up the drives that are connected, it should show the drive in question (assuming that you have it connected and turned on at the time of the scan).
What you're going to have to do is repartition the drive.
When your drive shows up in Disk Utility, it will appear in the left part of the window that displays all the disks and volumes mounted.
For the purposes of this discussion, I will define my terms. Volume means the physical hardware, Drive means the partitioned and formatted disk.
If you look at the Disk utility window, on the left are all the volumes and disks displayed. Those are what your computer sees when it scans for drives.
For example, on my computer I have 2 drives connected to it.
Each drive has a drive name, and a volume name.
465.8GB WDC5000 (which is my internal HD/hardware/VOLUME)
500 gb HD (which is the name of the DISK as it appears on my desktop)
1.4 TB Seagate (Hardware/VOLUME)
Freeagent Xtreme 1.5 TB HD (which is the DISK name as it appears on my desktop)
Now, you'll want to click on the VOLUME/HARDWARE icon in the left side of the Disk Utility window. When you do this, and you've clicked on the correct icon, you'll notice that it changes the menu on the right side of the Disk Utility window. So you'll know you've clicked on the correct icon, the menu should show five options.
First Aid, Erase, Partition, RAID, Restore
Having selected the VOLUME in question, you'll want to click on PARTITION.
That will take you to another screen that says VOLUME SCHEME
Assuming that you're only going to be running Mac OS X on this drive, I will tell you to create only (1) one partition. To the right of VOLUME SCHEME you'll see VOLUME INFORMATION. Put in the appropriate information. Select the correct format you want. If you're going to only use this drive on a Mac, then select MAC OS EXTENDED (JOURNALED) and click the APPLY button at the bottom right of the screen. This will repartition your drive and do a low level format. I am assuming that you have no data on this drive, thus there is no need to back up the data.
Once that completes, you'll want to select the disk icon from the left side of the Disk Utility window. The menu on the right will change. It will only display four options. They are: First Aid, Erase, RAID, Restore.
You'll want to click on ERASE
In the next window, it should say: Volume Format: Name:
Select MAC OS (journaled) for volume format.
For Name: put whatever you like
Now click on the ERASE button on the right part of the screen.
Once this completes, you should be good to go!
If you're still having problems, you can e-mail me
and I will try to help you out.