Oh, dear, you are in a state 
I'm generally in favour of dobbing frauds in. I think it's a nasty, sneaky kind of crime and it always hurts someone.
That said, I see an awful lot of posts here from infuriated 'friends' of benefit claimants, detailing lavish benefits they simply cannot be claiming - the 'friends' are so blinded by envy, they make up ridiculous scenarios and won't be told they've misunderstood something. I feel sorry for the DWP employees who have to process these mad reports.
I'm afraid your envy has taken you down a similar path, OP. If your mate's actually told you, in so many words, that the service she receives is being charged to other customers, then yes, there is fraud - or theft, I'm not sure which.
But has she told you this, exactly? Could the friend with the company simply be doing her a favour? In business, there's an informal concept known as a favour bank: people do each other favours, expecting to even out over the course of time.
Do you have reason to know the workers' payments are being fraudulently accounted for, or have you chosen to assume it?
In any case, I don't know who you'd report it to. Not the company, obviously! The local authority, perhaps? After all, you are not the one suffering detriment here (if anyone is) so you're not what the courts would call a complainant. Hmmm.
Perhaps you need to make friends with this construction company chap ...