like I said, I'd get legal advice. Should they seek residency in Australia for his wife, the application will show that they were never in the US. This would then beg the question as to how they have a US marriage certificate if they've never been to the US. Then if it's unlawful to be married online in Australia (which it is), it could invalidate their marriage in Australia and undermine their residency application. Having worked for the Australian Government and having also been through this process, I know they go through applications with a fine tooth comb, especially considering Thailand is a high risk country for Australian immigration. Not sure how Thai authorities would view this though.
not necessarily. Read the second bullet point. As you cannot get married online in Australia, it is likely an online marriage in another country would not be recognised in Australia. I'd be seeking legal advice.