I have been using a 2GB Memory stick pro duo on my k750i for the past 6 months or so...
It is an original Sandisk 2GB Memory stick which I got with 5 yrs warranty for Rs 2.3k..
I purchased it from a decent shop at Lamington road.
If anything goes wrong with the memory stick then I can get it replaced on the spot.
But frankly speaking, I hardly faced any problems with it..
Now if u are purchasing the memory stick from ur friend, then make sure that ur friend is providing u with an original Memory stick pro duo...If its Sandisk, then go for it without any hesitation..
Be careful...in the market, u get fake Sony memory sticks at throw away prices...If ur friend has got a memory stick of that kind...then ur phone will surely face problems like rebooting(and probably thats the reason as to why he wants to get rid of it..hehe

...)
As far as I know, fake Sandisk memory sticks are not that common in the market(as compared to Sony)
The best thing would be to try out the memory stick for a day or 2 on ur phone and check it for urself..
One way to do it is as follows:
Load the memory stick upto the brim (ofcourse keep a safety margin of 25 to 30MB) preferably with mp3s..
Open ur media player and open all files playlist and try to see if u are able to play the last song (or the latest song that u have added)
If it is able to play it then its well and good...otherwise its dupe!
Some dupe memory sticks can be really frustrating..
For eg: U may end up getting only 1GB on ur 2GB fake memory stick.
I bet u won't find any such problems if ur using a Sandisk memory stick..
I've tried using fake Sony as well as original Sandisk..
My phone works much better with a 2GB original Sandisk as compared to the 1GB fake Sony memory stick...
So be safe and be original.
If ur friend has got the memory stick pro duo with bill and warranty, ask for the bill and the cover (which is also equally important)
And also ask him from where has he purchased it, so that incase if u happen to face any probs with it, u can go to that shop and get it fixed...
Good Luck