I have concerns about a home educated child in my village. I don't want to go into details as it might not be appropriate. The concerns related more to benign neglect so not awful but enough to make me ponder and nag at me. If this child was at school I might mention it in confidence. What is the process for a home ed child if it becomes more than a nagging doubt?
If you are concerned about a child's welfare, it's the same route for a home educated child as for one at school: report it to Social Services. If your hunch is correct that it is "benign neglect" rather than actual neglect then it isn't likely to meet SS threshhold for intervening. It may just be a different parenting style.
If you are worried about a child's education, in the case of a child you don't know well, it's unlikely that you are in a position to know how effective it is. Home education can look entirely different from school, and yet work very well. For example, some home educated children including mine do no formal lessons at all and acquire skills such as reading whenever they are motivated to do so, rather than being taught at a specific age.