This is a really complicated topic. It's happened to me twice, but in completely different ways. The first time, the author I was critting thought herself generally above my level, so whenever I suggested anything she didn't like, she just chalked it up to me not being a good writer and ignored it. In the end, she completely stopped helping me as soon as I finished her ms, even though she'd only gotten through half of mine. It doesn't sound like this is what you are going through, OP, but it is important to both figure out what you want. If you don't think I'm good enough to help you with x y and z, what do you want? (I know this has already been said, just being thorough 
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The second is a young writer who tells a great story, but ignores the general rules of writing fiction. I know, what is a rule, but there are some things that most people who write agree on, such as not using modifiers, -ly words, when a stronger verb is available or when the dialogue makes it clear. When someone ignores something that you know isn't necessarily a SET IN STONE rule, like grammar, which isn't even totally in stone, but you know pretty much any agent will ignore the ms because of, it can be really draining.
I feel kinda like I'm merely wasting everyone's time with this post, but I guess the message I want to get across is I feel ya, and I think almost everyone has experienced it. Where you draw the line is something you need to decide on your own. Do you keep pointing out that there is a place and their is possessive, or do you focus on the larger things and just let them figure it out?
P.S. Having an author who doesn't take your advice isn't someone who takes you advice too much and basically doesn't want to do the time-consuming and usually painful job of line editing a rough draft. Those are the worst!
)The second is a young writer who tells a great story, but ignores the general rules of writing fiction. I know, what is a rule, but there are some things that most people who write agree on, such as not using modifiers, -ly words, when a stronger verb is available or when the dialogue makes it clear. When someone ignores something that you know isn't necessarily a SET IN STONE rule, like grammar, which isn't even totally in stone, but you know pretty much any agent will ignore the ms because of, it can be really draining.
I feel kinda like I'm merely wasting everyone's time with this post, but I guess the message I want to get across is I feel ya, and I think almost everyone has experienced it. Where you draw the line is something you need to decide on your own. Do you keep pointing out that there is a place and their is possessive, or do you focus on the larger things and just let them figure it out?
P.S. Having an author who doesn't take your advice isn't someone who takes you advice too much and basically doesn't want to do the time-consuming and usually painful job of line editing a rough draft. Those are the worst!