I think your husband sounds very wise. He is right, homework and school schedules and pressures in high school are more difficult. It sounds like he is preparing her mentally for the challenge and letting her know what the realities are. Sounds like she is mature enough to handle it, I am also a black belt and national champion and junior Olympian, so that's a big thing for her to accomplish and if her sensei was anything like mine, he/she has probably given her the grades talk as well, or will very soon.
Now, for him to give her this talk shouldn't backfire nor make her feel tons of pressure like you are suggesting. If he starts to hound her often it will, and if he belittles her for not having perfect grades, then yes, that will be a problem to address, but having honesty that the times will get harder for her is wonderful. I do agree to not include your name in talks that he does himself, but I am sure he assumed that you both would feel that her maintaining her grades are important.
In fact, I was a straight A student and perfect attendance until about 9th grade. Then my grades starting slipping and it did effect my gpa and college applications... and I was a very involved student, even in AP classes, thespian, varsity cheerleader, christian club. So, I'm going to side with your husband on this one.
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I also agree with JoAnn C about undermining your husband. It's better to get the story straight and say, "I'll have to ask him about it" before you make assumptions like "I'm sure you misunderstood", it only adds confusion and a power struggle that kids pick up on.
How Amanda W. responded was perfect. Your reassuring her that you value her trying and hardwork over a letter grade was wonderful though.
I also don't agree with moms saying he was out of line with this talk. It's not like it's the sex talk or he was abusive in any way, a father has the same right to be concerned and set expectations than a mother has.