This is a ridiculously misunderstood subject.
If your friend was providing the aircraft and you worked for your friend, and your friend had operational control over the airplane, you would be approaching the point where a 135 certificate would be required.
You are able to accept compensation to fly your company personnel around, correct? You hold a commercial pilot certificate and are accepting a wage for flying your boss? You can do this; you are a commercial pilot, entitled to receive compensation. You can receive compensation for flying the airplane from A to B, while your boss provides the airplane and doesn't accept compensation from passengers or for carrying cargo, from one point to another.
If you flew your regular airplane for your boss, the company airplane, and your boss invited a passenger to go from A to B and charged a fee, that would be an illegal charter. If your boss accepted a package to take from A to B for a fee, that would be an illegal charter. If your boss invites six of his best friends along and doesn't charge them, then it's not an illegal charter. You must consider who has operational control over the airplane (who provides it), and for whom do you work? If you work for your boss, and he has operational control, and the airplane is used only for his business, without charge for passengers or cargo, then you're legal.
Your airplane is down. You cannot fly it. Your boss obtains another airplane to use...the situation hasn't changed. He is still the operator of the airplane, he still has operational control, he still arranges the flights, provides the airplane, and you still work for him. Same as before.
Let's pretend you work for someone else. Your boss either owns the airplane or leases it, but either way, has operational control. He hires you to come do some contract flying for him flying him from A to B with six of his friends. Still legal; you're a commercial pilot, and you may fly for compensation or hire. The aircraft is not being operated for compensation or hire, and you do not have paying passengers or paid cargo on board. You're legal, no different than before. The fact that your boss owns the airplane, or the fact that he's leasing the airplane is of little significance to you. It's your boss providing the airplane, and the airplane is not for hire. If the boss is paying someone else for the airplane, either lease payments or bank payments to own, it's not your concern, and this does not make the airplane a compensation or hire situation...the test here is if the airplane is being OPERATED for compensation or hire. It's not.
Okay, change again. You work for your friend. He pays you. He provides the airplane to your "boss." Your boss decides when to go and when to stay, but you're straying very close to a 135 operation here; when the airplane and pilot are being provided by an operator, the line of distinction between charter and not is blurred. You're doing a charter.
In this case, however, you work for your boss. He's obtaining an airplane for his purposes. He might pay for gas, he might provide sexual favors in return, he might sell his first born. It really doesn't matter for the purposes of legality with respect to issues of charter. Your boss is exercising operational control over the airplane, you work for him, and you're in good shape. Your boss can make whatever arrangements he wants, so long as he's renting or leasing the airplane for his own purposes, and he's not operating it for hire.
If you rent the airplane and then carry your boss, you're then receiving compensation for flying the airplane, and receivving payment for the airplane; a strong case could be argued that you're exercising operational control and that you're arranging the flights, and then it becomes a matter of you doing illegal charters. You don't want to go there.
The best arrangement is for your friend to lease the airplane to your boss. He can do it by the hour, by the day, month, whatever he likes. So long as your boss doesn't operate the airplane for compensationor hire, then the money he's paying to have use of the airplane isn't relevant.