I don't know. They're Christian but their argument is more nuanced than just thinking meditation is opening up to the dark side. I think it's worth talking about, especially given the experiences shared by other posters in the comments.
Is it? Viewed through that lens, it doesn't seem like there is a lot of nuance.
Again, this is _your_ interpretation. They only said they're Christian, and you've decided this must be the reason they're against, and their opinion is worthless. You're just colouring their opinion with YOUR preconceptions.
Their opinion matches my experience, and other commenters, and I'm not Christian at all. Why are you taking it so personally? It's an intellectually stimulating topic, you're free to disagree without dismissing the person based on their religion.