Structuring, as noted in another answer, involves breaking up cash transactions to avoid the required reporting limits.
There are a couple of important things to note.
- Your bank is required to report transactions that exceed the limit ($10k, if my memory serves)
- Your bank may at its discretion report smaller transactions that appear to be questionable
- Simply reporting the transaction does not automatically bring any enforcement action unless other activities warrant more scrutiny
And, the biggest caveat - there have been many cases of perfectly legitimate transactions that have fallen foul of the reporting requirements. One case springs to mind of a small business that routinely deposited the previous day's receipts as cash, and due to the size of the business, those deposits typically fell in the $9,000-$9,500 range. This business ended up going through a lot of headaches and barely survived. Some don't.
A single batch of transactions, if it is only 2 or 3 parts and they are separated by reasonable intervals, is not likely in and of itself to be suspicious. However, any set of such transactions does run the risk of being flagged.
In your case, you also run afoul of the Know Your Customer rules, because it's not even you depositing the cash - it's your friend. (Why can your friend not simply write you a check? What is your friend doing with $5k of cash at a time? How do you know he's not generating illegal income and using you to launder it for him?) Were I your bank, you can be very certain I'd be reporting these transactions. Just from this description, this seems questionable to me.
IRS seizes millions from law-abiding businesses