Hi sturmae
I'm not familiar with the particular filter you have, so let me explain it another way. Some of the filter media is more coarse or fine than others, but they all have surfaces. The bacteria clings to all these surfaces, so some media will have more bacteria on it than others because it has more surface area. There is no part that has better bacteria, just more or less of it.
Keeping this in mind, you won't want to take so much of it that he doesn't have enough left to support the fish in his tank. If he gives you 1/4 of all his bacteria, they will reproduce quickly and no harm will be done. If he were to give you half of his media, there would be a risk that his tank would go through a mini cycle, which is not a good thing to happen.
Bacteria laden media, even a little of it, is valuable stuff. If you are planning to do a fishless cycle, you will basically be feeding it artificially with ammonia and it will reproduce itself. Eventually you will have grown enough to stock your whole tank with fish all at once. Alternately, you could skip that step, since your tank would be immediately cycled when you put the bacteria into it, in proportion to the amount of bacteria you have added. It's actually the filter media that you cycle, not the tank itself.
Since I don't know if you already have fish, or what your plans are, or even how much bacteria you will be getting, let me explain this a little further, just in case you might need the information. As a rough guide, if his tank holds 20 guppies (for example) and he gives you 1/4 of all the bacteria from his filter, if you put it in yours and immediately added 5 guppies, they would be fine. If he gives you 1/5 of it, you could add 4 guppies to yours, etc. In a week or two you could then begin adding a few fish at a time and the bacteria would reproduce to keep up with the increased bioload. If his fish are bigger, it would still work more or less in proportion to body mass.
I hope this helps you.
