Help + in + v-ing or Help + on + v-ing?
I used to help my friends in coding C++
I used to help my friends on coding C++
In this case, on doesn't work. In won't seem too strange, but it doesn't seem natural. With could work better.
Using the simple present rather than the progressive of the verb would seem more natural to me, without needing any preposition:
"I used to help my friends code C++"
I'm not 100% sure about generalising this, though. Certainly different verbs in the place of coding would work better with 'in' than in your example. I'm struggling to think of any where 'on' would work at all, but I hesitate to say there are none just because I can't think of any.