Lifestyles not matching income and not in and of themselves evidence of a problem. People may have family money, had other successes, partners or parents making substantial contributions or even just choose to overuse their credit cards or home equity lines. The later may raise questions about judgement and discretion but it is not an indication of malfeasance.
The rest of the situation gets tricky. If your cousin is being asked to falsify, hide or obscure material information from the Board, other members of senior management or investors at the least he needs to document that he raised the issues with the managers whose decisions are causing him concern.
Depending on the scale and details of the issue it may be appropriate to notify either more senior managers (although in this case I suspect the discomfort arises with the directions of the CEO or equivalent), the Board or company counsel. Once he does this it is likely things are going to get uncomfortable and he should be prepared for unpleasant and tense relationships including possibly having to leave the company.
If he is being ordered to do something he considers improper, illegal or inappropriate he should document the request, the reasons he objects and, if the order is not changed, resign with a carefully drafted letter laying out the situation. He should always stick to the known, demonstrable facts and not speculate. He should not communicate with the investors (except through their Board representatives, if any). If he has prepared or signed any statements which he believes are materially wrong that have been given to banks or other lenders or regulators he should get advice about his obligations and personal exposure.
This is a difficult situation and before taking any steps he should make sure his concerns are not the result of miscommunication, misunderstandings, incomplete information about compensation agreements which have been properly approved, etc.