You don't say whether or not you are formally training with any of the Accountancy Institutes. If you are, then the the biggest question you should consider is whether by doing this work you are breaking any of the rules of your Institute. If you are, then you should steer well clear.
Assuming you know what you are doing there should be nothing inherently illegal in helping these people but, apart from the caveat above, you should really only commit to this if you have a good understanding of the tax & company law issues affecting this company.