OP, it's clear that you don't know how to communicate with written form only. Yes, you might have had good intentions with wanting to give advice, but it came across as very rude. If you had spoken that way to me in a run, I would have ignored you, at best.

You have to understand that in a text box, they can't see your face, hear your voice, or feel a tone or expression. All they have is the literal words, and that's it. Unless you make a very direct effort to chose your words carefully, it will come across as rude.

Next time, perhaps start out with a question, and/or a compliment. Maybe something like:

You: Oh, cool, a BLM! I love having a BLM in my party!
Them: [misc smiles and gratitude]
You: Hey, I noticed your spell order. May I give you a tip?

At this point, wait for their answer. If they say "No", let it go. It's not your place to correct the world. If they say "Yes", start with what they did right, then add what they can do to improve.

Either way, you need to be far more mindful of how you use a text-only form of communication.