It is said that one should ones friends close and enemies even closer. Because it is easier to keep an eye on your enemy where he is close to you.
Dont know how many realize that one of the most important jobs of an embassy is Spying. Diplomatically it is called keeping mother country informed of the developments in the host country. Thus for the Indian High Commissioner to invite current and ex chiefs of ISI is well within the established norms of diplomacy. For ISI chief to accept such invitation is nothing but a PR exercise.
The question is what would have happened if Pakistans High Commissioner in Delhi does the same to the head of RAW? He probably does so any way, except that no bigot such Hamid Mir is present to make such a big thing out of it.