Is it reasonable to evaluate spelling/grammar in reports for students diagnosed with dyslexia?
I’ll go for a legal answer, at least valid in my current place of employment (France): dyslexia, which must be medically characterized as a learning disability for an individual, is covered by the laws regarding disabilities. As such, students impacted can request that exam conditions be modified for their benefit: most commonly by giving them additional time to “compensate” for their disability, but it could also include changes in setup (such as having access to a dictionary, spellchecker, or even a secretary). On the other hand, no leniency should then be granted on judging the exam paper.

The operating principle in all cases should be that of equal opportunity. (See here a summary of jurisprudence by the Association lyonnaise de droit administratif.)