##By having him claim a long list of medical conditions## ###Photosensitivity### There seems to be a medical condition for everything under the sun. In this case there are a number of medical conditions where one of the symptoms is the person's skin becomes sensitive to sunlight. A person can even be allergic to UV without a specific medical condition causing it. You can even have him claim a medical condition that his eyes are extremely sensitive to light (Photophobia), and so does not like any strong light source.
###Garlic Allergy### People can be allergic to garlic. Symptoms can even be induced by inhaling garlic dust, so needless to say he tries his best to keep his distance from it, lest he wants his throat to swell shut and die. In his case I would recommend explaining that he is allergic to a chemical in garlic which can also be found in onions and leeks, so he avoids those too. Having him freak out over a sight of a leek when it is of no threat to him helps build the charade, since people know vampires are not allergic to leeks.
###Cannot eat solid food### There is even a medical disorder for being unable to eat solid food: eosinophilic gastrointestinal disorder. In short the body triggers an immune system response when trying to eat solid food. Most common result is in vomiting up the solid food.
###Having Fangs### Porphyria and a number of Ectodermal dysplasia type syndromes can result in the classic vampire teeth. However there is a slight problem with this:
- has a lot of money and doesn't have to work.
The witch hunters will question, if he is so rich that he does not need to work, then why does he not see an orthodontist to get his teeth fixed? My recommended response would be to say he does not care: it does not impact his ability to eat, and he does not care about his smile, therefore why bother getting it fixed? Other reasons for not getting it fixed includes: he really really does not like dentists let alone orthodontists or he claims most forms of anesthesia do not work on him (and in some cases even makes the pain worse) and as such it would be a rather painful experience to get it fixed so he has been putting it off, but will eventually get around to doing it.
###Reflection & Mirrors### For this answer I am assuming devices like cameras can still take a picture of him, otherwise it would be very difficult for him to explain why he and no one else have any pictures of him and you would then need to consult this question on Plausible explanation for lack of reflection in a mirror and reflect on the answers to it for help.
Mirrors and reflective surfaces can be straight up evil for someone with sensitive skin. Dad tells the following story to guests he hosts:
One day he was minding his business in his house and took a nap in his living room recliner. He wakes up to find the whole side of his face and right arm inflamed and covered in a rash. He realized that a neighbor had park their truck, which had lots of chrome on it, on the street that day. The sun just happened to be at the correct angle while he was sleeping to hit the chrome and shine a beam of daylight briefly into the house and hit his arm and face. The rash took days to go away and ever since then he absolutely despises everything that can reflect sunlight.
The story of course is a total lie, but he can use it to explain why the only mirror in his house is in the bathroom, why all of the cutting knives in his kitchen are ceramic, and why he avoids cars with lots of chrome on them.
Because of his supposed skin condition it would be normal for him to always wear clothes that cover as much skin as possible and hats/hoods that provide as much head covering as possible. So his face would be the only thing at risk of being caught in a mirror. Since his face would still be invisible to a mirror he will need to be vigilant to spot mirrors and reflective surfaces and see if he spots anyone else in the reflection. If he can see their face in the reflection then that person will be able to see his lack of face in the same reflection.
Additional things he can do to help avoid reflections include:
- Have him like using plastic utensils and paper plates, to help avoid additional reflective surfaces.
- His car would have dark tinted windows so other drivers cannot see who is driving the car if they look at his car through one of their rear view mirrors. This also may provide the necessary protection if he has to do an emergency drive in the day.
- Avoid public restrooms (do vampires even need to use the restroom?)
###Conclusion### Some people seem to struggle with lots of medical conditions. Have the vampire dad do his research and fake all the symptoms needed to make it look like he has the above medical conditions and his daughter will likely believe it. It may even result in her being quick to defend her poor suffering dad from such allegations of being a vampire, since they do not understand him like she does.
Humans are good about rationalizing things and will tend to assume the more plausible explanation is the most likely one. As a result who would she believe: Her dad saying he has eosinophilic gastrointestinal disorder or a stranger saying her dad is a vampire? After all vampires do not actually exist... right?
Edit: Added sections on Having Fangs and Reflections & Mirrors