If you want to sponser it do so. Maybe work with the school so that you can pay anonymously for 1/2 and they or you can give Jon a letter saying that 1/2 is paid for this trip, he only needs to pay the other portion to attend. This way the money dose not even end up with the parents or even the child but for the real purpose of going towards the trip for this specific student.
If I was in the situation and could afford it I would help my daughter's friend, but I would do the above. Some people are just not repsonsible with money so if I want to sponser or help out I usually pay for it up front or ask to take the child with me to do such and such (bowling, skating, shopping whatever). This way I know for sure the money is getting used for what I attend it to go towards.