You probably start by asking, "Should my dialect coach be the one I even ask to help with dialogue coaching?"
The obvious answer... YES!
Here's why? Because outside of helping you to seamlessly incorporate your dialect into your acting and helping guide you in making the specific dialect choices that equal specific acting choices, your dialect coach specializes in how the language your are speaking (i.e. Dialogue) is put together for dramatic effect. They do this because how the language is spoken and put together is an intrinsic part of a dialect of that particular language.
Let's go a step further... I've said before that you come already pre-loaded with a dialect. But you also come pre-loaded with a particular and idiosyncratic way of speaking that dialect that is unique to YOU. We call this your idiolect. If we were to write your conversations down, each participant in the conversation - including you - would have a slightly different way of choosing words, which slang and word order to use, and speaking and constructing their arguments and interfacing with the world. This is in addition to the sounds of the dialect itself!
Your character comes with this feature as well.
Your dialect coach will work with you to incorporate this level into your acting as well. Remember, this is NOT helping you with line readings or telling you "how" to say your lines.
Think of it this way, we integrate the dialect into your brain so you can speak extemporaneously in that dialect without thinking about having to "think" about how to make each sound change. If we get into the nitty-gritty of how the dialogue is put together as well and integrate the way of speaking (and THINKING) into your actor's brain, then you can start THINKING the thoughts that create the dialogue in the dialect.
This is a HUGE step in making the dialect a part of your transforming into the character, and you'll love it as an actor when people meet you after seeing your film and you act and sound so completely different. They'll stand amazed at how you were able to transform into the character! It can be a huge, extremely fun, and really great part of the magic of acting that you love to share with your audiences.
So bring your dialect coach in on your process at the beginning - you won't regret that decision at all!!
Anyway... Those are my thoughts!