Update on the post that gave the nice MS link to download the Windows 7 Professional 64-bit .ISO with Service Pack 1 included..... it is not for use with a HP W7Pro64 sticker COA or HP OEM COA, but there is a way around that for what I call the "HP sticker COA" situation, as you'll see. That Microsoft web site is HERE.
You have to enter a valid 25 alphanumeric code, which is 5 sets of 5 alphanumerics, at that MS page. Then the page checks to see if your code is valid for download before it allows you to download. If your code is from a vendor such as HP it tells you that you must contact the vendor and will not let you download the ISO. Same if you enter the standard HP OEM 25 alphanumeric that your workstation will have if it is straight from the factory, or if you have used the Restore media to re-create an official HP OEM install. Here is what that site gives you if you enter the HP sticker COA:
That uniform HP OEM COA code is 74T2M-DKDBC-788W3-H689G-6P6GT. This is no big secret, and that code will only be on a build from the factory or on a build created from the HP OEM Restore media, not on a build made from a "System Builder" retail Microsoft W7Pro64 install kit.
In the same way, you can't take a HP sticker 25 alphanumeric COA code and use it on a non-HP computer, of course.
But, if you get a 25 alphanumeric W7Pro64 COA code from a Microsoft System Builder (retail) kit you can plug that in to that site and it will let you download the ISO, and then you can use that ISO to install clean using your HP sticker 25 alphanumeric on your HP workstation. That is a mechanism to get the more generic installer and use that HP sticker COA legally. You already bought that sticker COA with your workstation, so it is OK to use that license code.
The upside of using the Restore media with its HP OEM COA is that it includes all the correct HP-selected drivers (which you'll likely want to update anyway for a good number of them). That also lets you clone a build and have it work in another same-type HP workstation, making cloning quite easy. A clone of that will not work in another non-approved HP workstation. It knows what it is going into. For example, I would not be a bit surprised if a version 1 Z600 clone would not be accepted as a clone build in a version 2 Z600, but have not tried that.
So there........ I hope that helps. For the OP..... borrowing your friend's Restore media seems OK, and I'd archive those DVDs for the future. You'd want copies of all the DVDs.... one may be a "starter" DVD you only use to unlock the mechanism. You don't have to install all the stuff that HP offers on those, either.... at least mine for the Z620 let me pick and choose some.
Stay legal..... it is not OK to use such media unless it also would have come with your Z600. Most would have had that, however, but if your friend has a version 1 Z600 and you have a version 2 his media may not work for you, and the reverse is true also. If you are able to do a Restore from it you likely will be able to make a Restore media set from that install on your Z600. Capture that on a quality 8GB USB stick if it will let you.... much more handy.