.: I haven't passed this past legal review so for what its worth: Part of my standard requirements for everyone include income is at least 3X rent, minimum credit score, say 640, Clean record and no evictions as well as deposit is paid in full at signing of lease and 1st month rent is paid in full prior to handing over the keys.

Consider Hiring a Legal Expert- German property law can be complex.

Note that legal and transfer costs are not deductible but can be added to your cost basis.

In total, including money lost from rent, legal fees, and repairs, the situation cost almost $30k.

Much better than a legal filing or media attention. 

Or a legal perspective?

Your client should consult a real estate attorney to draft an agreement with the ex and ensure there are no legal or tax issues, especially since this is an investment property.

Seek legal advice, however, what I understand is that there are really two way to wholesale that is legal in the state of Arkansas.

Is there any legal action one can take or is it just a lost cause?

The real prohibitive costs come from the legal, compliance, and reporting requirements of registering a security.