Another one with young children who wouldn't book a BnB. We tend to book self catering caravan or holiday cottage.
I book self catering cottages and caravans for the space, the kitchen, the fact that we can put the children to bed in a separate bedroom and not have to sit in the dark/ in silence. Not having to worry about cleaners in the room or feeling like we have to be out most of the day.
I need flexibility for meals. DD is a picky eater and a grazer. Sometimes all she wants for tea are simple bits like toast and beans, spaghetti hoops, sandwiches and fruit and veg sticks ect, all which can be made at home for 1/10 of the price of going to a cafe and can't be made in a BnB without a kitchenette. I also need somewhere and somehow to wash and sterilise baby bottles. DD drinks milk at bedtime still and doesn't like UHT milk.
A separate bedroom is a huge help as otherwise the children go to bed at 8 and me and DH have to sit in the dark/ in silence and even then because we are there DC do not sleep and instead find 30000000 reasons to stop up longer. Many bnbs cram the beds in 1 room, no- one sleeps well because every time someone goes to the toilet the whole room is disturbed, DD will get up at 5 (really not other guest friendly) and put the TV on or start crashing about with toys - again waking everyone up. Then be raring for breakfast at 6 - again not BnB friendly.
Then there is the cleaning/ room service aspect where most bnbs expect you to have breakfast and then be out for the from around 11am so turn rooms round. My youngest naps at 11.30 till 1 and then 4-5. Sometimes if they get up early they nap 9-11 so we are often in the room. Eldest is then often stuck in with cabin fever whilst youngest naps, we usually book either a beach side family hotel or a caravan on a caravan site so DD can go to the park/ beach to burn off some steam whilst youngest naps.
Similarly space, BnB family rooms are generally cramped so play space is limited.
Personally I would advise your friend to stick to the adult market. Look at what everyone else is doing, serve food quality nice breakfasts, if they want to stand out go quirky but adult focused.