Overall, it can be a profitable business, but most people get into it to become an investor and 99 percent fail right away because they don't know the slightest thing about repair costs, talking to sellers, and are only in it for the money and not to help people.

If he failed to pay, it could be construed to be a failure of consideration so the deed fails and she's still in title with him and all he can convey is his half. 

Some are basically renting out bed space and require tenants to sign an agreement to stay clean, and be tested upon request, with failing the test potentially leading to immediate eviction.

The reason why most investors fail is because they are not properly capitalized.

It can become a really big mess and lot's of them failed the last downturn.

whay you fail to understand is that I am currently the nation's expert in credibility and Conversion strategies.

You fail that test as well. 

Send them a short, professional notice that they failed to abide by the agreement and you are putting the rental back on the market.

Many fail to understand screening, managing, understanding the culture and treating people with dignity.  

Have them pay the repair company before closing with that much of your earnest money going hard in case you fail to close.