You absolutely run the risk of the accusation of structuring. One can move money via check, direct transfer, etc, all day long, from account to account, and not have a reporting issue. But, cash deposits have a reporting requirement (by the bank) if $10K or over. Very simple, you deposit $5000 today, and $5000 tomorrow. That's structuring, and illegal.
Let me offer a pre-emptive "I don't know what frequency of $10000/X deposits triggers this rule. But, like the Supreme Court's, "We have trouble defining porn, but we know it when we see it. And we're happy to have these cases brought to us," structuring is similarly not 100% definable, else one would shift a bit right."
You did not ask, but your friend runs the risk of gift tax issues, as he's not filing the forms to acknowledge once he's over $14,000.
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.
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.
There's a difference between your street level drug dealer sending you sales proceeds of $20,000 in $5,000 increments to avoid sending you $10,000 or $20,000 at once to avoid the scrutiny of a government agency that might not be thrilled with your business venture, and a tire shop paying a wholesaler $5,000 each time funds are available up to the amount owed of $20,000.
The former is illegal for a few reasons, and the latter is business as usual.