--ddl
drop table orders
drop table persons
go
create table persons (pk varchar(10) primary key)
create table orders ([order] varchar(10) primary key, person varchar(10) constraint fk references persons(pk))
go

--sample data
insert persons(pk)
select 'PersonID1'
union select 'PersonID2'
union select 'PersonID3'
go

insert orders(person,[order])
select 'PersonID1','OrderID1'
union select 'PersonID1','OrderID2'
union select 'PersonID1','OrderID3'
union select 'PersonID2','OrderID4'
union select 'PersonID2','OrderID5'
union select 'PersonID3','OrderID6'
union select 'PersonID3','OrderID7'
union select 'PersonID3','OrderID8'
go

--create a temp table to work with
select
    person,
    [order],
    space(8000) as list
into
    #workingtable
from
    persons p inner join orders o
        on p.pk = o.person
order by
    person,
    [order]

--some variables
declare @list varchar(8000), @lastperson varchar(10)
select @list = '', @lastperson = ''

--build the list column of the temp table
update #workingtable
set
    @list = list = case
                       when @lastperson <> person then person + ', ' + [order]
                       else @list + ', ' + [order]
                   end,
    @lastperson = person

--return the csv
select
    max(list)
from
    #workingtable
group by
    person

drop table #workingtable
go