I generally agree with this but I would have it be more nuanced. Right now, we don’t know how the student’s B+/A- grades compare to those of the student’s school cohort. At most schools, your statement would be true, but if the OP’s kid is at a school where these are the top grades in the class, then there’s more wiggle room.
Generally speaking, I think that the odds of getting an acceptance via ED are best if a student is no lower than the 25th percentile of the college’s admitted class (in terms of GPA, class rank, test scores, etc). If one is below that 25th percentile, I suspect that ED is not going to be a big enough hook to gain admittance, barring something like an exceptional essay that hits the right reader in just the right way.