Dividing this into two parts:
1. Can one person own the roof? Yes they can – as you said, the previous owner of the entire building chose to include the roof (or most of it) as part of the title to his lot when he subdivided the property. That was his prerogative, and anyone buying into the building did so on that basis.
The strata doesn’t own the roof (most of it) because the previous owner included it as part of his title.
On the plus side, depending on how the strata scheme is set out and what the by-laws say, if one person owns most of the roof, they should be responsible for the ongoing repairs and maintenance of that part of the property.
Do the by-laws make any provision regarding that? If not the other owners may have a case for having a by-law that deals with the issue, or perhaps reallocation of the unit entitlements to reflect that the owner of the roof has the sole benefit of it.
2. In terms of the small access area on the roof to which the other owners have access – you don’t say whether that is common property. If it is, the owner of the rest of the roof can’t insist on bricking up the view, because it isn’t part of their property. They could only do that if all the other owners agreed.
However – if it is an access arrangement by way of something such as a licence for the other owners, and the roof owner actually owns that part of the property then they might be able to insist on bricking up. It sounds unlikely that is the case, as you say there is a boiler room and laundry area which would usually be common property. If you have more detailed information that would be useful.