Generally speaking, it is not illegal. This is against academic norms, but it is not illegal. Helping others with their papers belongs to helping others. This can completely charge a certain fee, and it also belongs to a small contract between two people. As long as two people discuss it, it is not illegal.

It is illegal for the two men to reduce the weight of their papers without consultation, and the expenses involved are relatively high, so things cannot be properly solved. They can safeguard their basic interests through legal rights, but only if their actions are not illegal.