I'm sorry that your dad is having this issue - it's not common for a female cat to spray.
Whenever a kitty is peeing out of the box or spraying, I'd always start with a vet visit to make sure it's nothing medical. UTIs and other bladder issues can result not just in inappropriate elimination but in spraying as well.
I will say, though, that since it seems to be ongoing, there's a good chance that it's behavioral. I take it kitty is still spraying, even though the other kitty is no longer there?
Stress is probably the most common reason for spraying (with girl kitties anyway). It could have started out as stress due to the other cat's presence. Even if kitties seem to get along, there can be issues that we don't see.
It could also be a kitty that wanders by outdoors that she's seeing or smelling. Is the TV near a window?
Since your father doesn't have a strong sense of smell, she's probably sprayed elsewhere, but the TV is where he noticed it.
Anything he can do to reduce her stress will help. Despite the fact that she's the only kitty, she may not really feel like it's her space yet. Spending extra time with her, giving her attention, playing, and providing a few "safe" places - a cat tree, little beds in quiet places could make her more confident.
Getting a pheromone diffuser (like Comfort Zone) and/or putting some homeopathic drops (Bach's Rescue Remedy, Homeopet's Anxiety Relief) in her drinking water or wet food can also help.
I'd recommend getting a good black light and using it throughout the house to identify any other areas that have been sprayed. All areas, the walls, baseboards, floors under the sprayed areas, should be treated with a cleaner specifically formulated to break down kitty pee, like Nature's Miracle; vinegar, bleach, other cleaners will get rid of the smell for humans, but not for kitties. And when kitties smell their own urine, they tend to go back to those same places.
The spraying is a sign that somehow she's not 100% comfortable, whether due to pain/discomfort from a medical issue or to stress/lack of confidence.
I hope he'll be able to figure out the cause so that his daughters aren't unhappy when visiting!