Last year, I was living with my mom while she was unemployed. She wanted to charge us 600+ for her spare bedroom, and utilities, which my husband - then boyfriend -and I agreed to pay. However, 3-4 months into this, she handed my husband a fistfull of unpaid personal bills and told him to "pay these". It wasn't a request, asking for money for help, etc. It was a demand. Of course, we moved.
This year, my mother has been employed and also found a room mate willing to pay $500/mo for her spare bedroom, and was getting back on her feet. Over the summer, I was helping my mother make some repairs and improvements to her kitchen. Her temporary room mate was moving out, and we were discussing how she would really rather live by herself. For me, this presented a good opportunity to broach the subject in a semi-passive way...
Stated that I would much rather live with room mates, that the money was worth the hassle. She insisted it wasn't. I asked her, is living alone worth $6000 a year, or more? She hesitated, but still insisted it was. I could have gone on to point out that having a room mate for just 3.5 years would pay off her delinquent student loans, another 1-2 years beyond that would pay off her car... Even though it didn't change her behavior, it was still a great opportunity to provide a different financial perspective.
The moral of that story being that if you display frugal and positive financial models to your dad through your own behavior, it will show him that something different is possible. I would be curious to know who handled the finances and bills while your parents were married - that might give a good indicator as to why he doesn't seem to know his limits or how to manage his income against his expenses...