Right, I'm not suggesting it, just wanted assurance that there was something preventing it.

They make what are called backflow preventers.

Unless you are there 24/7, its hard to prevent.

The best way to prevent unnecessary repairs is to check in on your tenants regularly.  

There's a lot of knowledgeable folks to learn from and I'm also happy to share the mistakes I've made to prevent it from happening to others.

Any suggestions on how to prevent UD tenants from trashing the place?

If so, would that prevent me from taking out another HELOC after I paid it back?

The CC&R clearly prevents it along with an acknowledgement of the rules when a tenant signs up.  

Have you looked into whether this will prevent you from taking a 3.5% down loan in the future? 

Sell the lead to a real wholesaler who will pay you a finders fee if you don't have time to learn to do it yourself.Since a friend is involved I would do a double close to prevent him from finding out how much you made him overpay for the property.