It might be a state/county thing, but I remember several instances where customers had Comcast for TV and Verizon FiOS for Internet. If I recall, FiOS had a fairly weak (and pricey) video package, yet Comcast didn't offer consistent Internet speeds. I'm sure the customers were paying more than using one provider would have cost, but they had priorities that warranted such. I can't see how either provider could argue this if the customer made it clear what their expectations were. I don't how their billing was set up though.

I'm more curious about how either provider in your case even knew about the other without doing some very intrusive and unauthorized digging around. It sounds like a case of sour grapes to me.