Think like a marketer, agency pal!
In most business practice, marketers are responsible for building a brand or product, and delivering business result. They come up with marketing plan layouting the objectives and measurements as "why", following with marketing ideas or tactics are, as "what", and finally the last part of "implementation" as "how" to deliver. Putting all these in plan, it takes a lot of efforts to sell through internals amongst operation, sales, products and channels for agreement or at least endorsement. It's hardly be a sole single point of approval in the whole journey.
In most of the case, the generation of creative and communication idea is at the implementation phase of a marketing plan, and that is mostly the external agencies, be it advertising or technical agency, are being engaged. If agency friend can understand the planning phase and it's the chain of approval or operation process of your client, it will make the "idea" be more effective for implementation, and easier for your marketing client to sell internally.
Following is a good article from Damien Cummings, global head of digital marketing for Standard Chartered Bank who gave great insights to agencies.