I would venture into guessing that the client wasn't told about this. If I was in a similar situation and I'd be told the company I'm paying to work on my project is outsourcing it, I'd think they're either a) they're too busy for me/I'm too small of a client for them, b) money is money so they're taking my project, but I shouldn't bet on spectacular results. Also, there'd be some broken trust involved. There would also be the risk of losing the client altogether the moment they hear you won't actually be working on their project, but someone else would be instead.
Most boilerplate consulting contracts require you to notify clients of subcontracting arrangements, and many BigCo client boilerplates forbid subcontracting altogether.
But so what? You talk to your client about a prospective engagement, then you build a "dream team" proposal that puts you on the hook for the performance of the whole project and makes you the single point of contact for all project matters, but then uses the specialized expertise of partner firms as a sales point. You sell with the subcontracting arrangement, not in spite of it. It's not something you sneak in under the wire.
It doesn't have to be nefarious. Sometimes there are certain aspects of a project that are outside your company's core competences. It's only ethical to pass the work to a qualified professional. Clients pay for results - it's a business transaction.
Really? I say "our agency is so busy at the moment that someone else will handle your project, but I will personally oversee and QA it. Our customers not only seem to not mind, but they're extremely happy after the project has completed.
I think you mentioning that you'll personally oversee and QA their project has a big impact on how positive the news is received. In fact, that's a great approach!
