My thought would be that she would be better to keep the box room and keep it as her ppr. However she needs to get on top of this situation where a girlfriend has moved in practically. I think she would be better renting out the other room to a total stranger and not making her ppr a very cheap house for the other two to come and go as they please.
Obviously all this depends on how long she has the house, what equity she has built up and if there is demand for rental in the area.
It sounds as if she is busy with home so would she have the time and energy to be a landlord? Also by keeping the room and using it, even if its only two days a week, she is there looking after her property and she has a bolt hole when she needs a break from home. She can earn a good lot tax free with the rent a room scheme and keep her interest relief. If she changes to a n investment situation what happens if she can't rent at all. With the rent a room scheme she also has more control over who lives in her house.
I must add that this is only my opinion and to be honest she should get professional tax advise before she does anything .