Is calling someone 'didactic' risking offense?

As I understand it, 'didactic' is used to mean something that has the ulterior motive of teaching especially in a moral connotation and also to mean patronising, or appearing patronising.

So does describing someone or someone's manners as didactic make it an offensive comment, or indeed simply mean they have the inclination to teach, moral or otherwise?

Greek being my mother tongue, I know the word stems from the greek 'διδακτικός' meaning 'someone who teaches' though in a non-professional manner, but I don't know if the word's meaning has been adapted through the language transition.

It is not clear from a dictionary definition, but it is usually employed in a negative sense. Consider some of these recent examples from the OED:

1979 J. B. Ingram Curriculum Integration & Lifelong Educ. iv. 66
This one-sided view..portrays teaching as didactic, theoretical, and ego-centric, the teacher being active, but the learner passive.

2009 S. Cross Adult Teaching & Learning i. 9 Techniques identified as teacher-centred have come to mean all that is didactic, boring, self-serving and neglectful.

2011 Daily Tel. 14 May 21 In drink the middle-aged male is a repugnant beast—bossy, loud, didactic and know-all.

I think the main problem is that it has come to imply a form of teaching in which the pupil plays little part but as recipient of information. And I suppose you could say that it is critical, if not mildly offensive to call someone's writing didactic

It's not a commonly used word, but if we look at definition 2, here (which you refer to with your comment about appearing patronising), we can see that it's quite insulting.

http://www.dictionary.com/browse/didactic

adjective

1.intended for instruction; instructive: didactic poetry.
2.inclined to teach or lecture others too much: a boring, didactic speaker.
3.teaching or intending to teach a moral lesson.
4.didactics, (used with a singular verb) the art or science of teaching.

With all language comprehension, we have to consider the motives of the speaker, ie what were they really trying to communicate? So, if I was a teacher, and one of my students told me that I was didactic, then I could, according to the definition, think "That's fine, she's simply saying that I'm a teacher". However, since we both already know that I'm a teacher, her motivation for saying it with this meaning doesn't make sense - she would just be stating the obvious, it's completely pointless.

So, I would then assume that she didn't mean definition 1, and that she probably meant definition 2: the motivation makes sense here, as she's telling me that I come off as patronising - she's either insulting me or she's trying to help me realise how to be a better teacher. So, I might be insulted, or I might "take it on the chin" and try to not be as patronising in future.

Either way, she risks offense (which is really what your question should be asking: deciding that something is objectively, inherently offensive can be difficult, and is prone to debate. It's more useful to ask if you risk offending people - that's a much easier question to answer.)

EDIT: this answer may appear didactic. Apologies if so.