I guess this would fall under a similar category about doing business with family. If you're around them and see how they put off work, real life or otherwise, then commissioning them might be a bad idea. And while it's nice helping out friends, if they have that sort of habit loaning them supplies, free services, and money would also fall under bad ideas.
It's very easy to nicely and firmly tell friends that you aren't able to help, or even explain why you're hesitant to help. As i believe a true friend will be understanding. Yes it might hurt their feelings a little bit and it's possible that they may feel you're being a BIG MEANIE for not being understanding but the same could really be said of them for not understanding you in return.

Generally, if the people i call friends have bad business practices, i don't do any kind of business with them. I'm still their friend but i don't mix the two.