It is natural that Tintin should offer to help the Father and hence the students, because he saved his life.
It is not unusual to find substitute teachers to fill in when the regular teacher is unavailable. Do Tintin's motives really have to questioned to the point that he should not have given the lesson and leave the pupils to their own devices rather than help improve their knowledge? (In passing I admit that the b&w version in which Tintin tells them about "their country" Belgium is disgraceful from a modern point of view.)
Are you saying that rather than cure the man's fever, Tintin should have just left him to suffer? That if he had not been the local "king" he would not have bothered? There are other instances when Tintin helps local people out of selfless consideration such as when he rescues Chang from drowning in "Lotus" or defends Zorrino from the local town brutes in "Prisoners of the Sun".
Once, when I was walking through London at about 5am (I was on the night shift at work), I saw a man being hit by a car which just sped away. I went over and helped the man to the pavement and called for help. Was I supposed to leave him there in the middle of the road in agony?
I recall once at school in England in the early 1980s, a teacher told us about how he once taught in an African school. He even showed us slides of him, a white man, checking the work of his black pupils in much the same way as Tintin. I remember asking him why he was doing it rather than a local teacher and he explained that he was offered the job because it involved teaching chemistry of which there was a shortage in that part of the world.
Does that make it alright or should the pupils have missed out on weeks of chemistry lessons until a local could be found?
It appears that you cannot help people these days unless you have pure uncontroversial motives which may not be good enough either. The reasons do not justify the ends.