If your company has a lump-sum number in the contract for services, then that's not a big deal for your company, though he is probably ripping off his own company. This is highly unlikely, though.
Most contracts are based on an estimate of hours worked, and have a provision for authorizing more funding when those hours run out. Padding his hours like that would accelerate that process and certainly cause your company to pay.
Also, even if he gets paid a salary from his company, consulting gold, where they get their money, usually, is by their employees working billable hours. He may not see more money, but his company is very possibly charging yours for his time, at a huge, massive markup over what he gets paid.
If your company is directly contracting with him, then it is a big deal. If he had a salary based on work completed, he'd be an regular employee, not a contractor, unless he is an awesome negotiator.
I'd at least mention it to management, and leave it at that. Something along the lines of "I heard contractor X mention he logs a full day with{"with his companycompany", if that applies}, even if he only works until 14:00, which I've seen that he often does. I don't know if we get billed by how many hours he reports, but if we do, I thought you might want to know about that. No need to follow up with me, I'm not personally invested in this."