It’s pretty straight forward I figure: If the post is readable and makes sense without needing to re-read it multiple times and derive an interpretation, rather than understand its meaning then, I figure, it will not be deleted (at least not due to grammar). If on the other hand one needs to infer a possible interpretation out of a range of multiple variants due to grammar construction, then someone may report it as low quality and, if the mod/admin agrees, it may be deleted.

In any case, it is way better to argue on the matter by example, so simply providing cases of your friend’s deleted posts (he can perhaps find then in his drafts), or simply indicating his profile name (that would enable us to try to find those deleted posts), would help to analyze concrete cases rather than generically scenarios where the degree of grammar accuracy/inaccuracy is important to tangibilize.