You asked your friend to do work and you allowed him to do the work: therefore, you have to pay the agreed-upon amount--i.e., for his workers. It doesn't matter whether they choose to bill you under her company's name or under his name directly: when you allow someone to do work for you, you have to pay them, both under a contractual theory (even if it was an oral, or unwritten, contract) and to avoid "unjust enrichment," or improperly benefiting by taking someone else's work without paying for it. Obviously, when you pay you should do so only after getting a written agreement that the amount you are paying is payment in full for all work done by "John Doe" and "Jane Doe, Inc." for your fence.It doesn't matter that you did not get firm or detailed estimate in advance: by what you have written, you agreed to pay the "labor for workers" without qualifying what that amount would be in advance. Agreeing to pay whatever that labor was, you have to pay it; and if you did not want to incur an obligation to pay without detailing it in advance, you could have refused to let your friend or his workers do the work. Yes, maybe it was a bad idea to agree to have work done without knowing the cost in advance, but that's what you did, by coming to an agreement with him and letting him do the work.It would be reasonable to ask for documentation of the labor costs and then only pay that amount, per what you had agreed.