At the time of the 1851 census, William was known as "James" which I'm thinking has to be a mistake. Susan is supposed to be 5 years old which I also think is a mistake. Son William is not mentioned but there now is another daughter named Firth. It doesn't say that Mary hailed from Orkney on that FamilySearch census
Susan is 4 months old on the 1851, (not 5 years) - which is correct.
Here is the 1851:
City Garden Row, St Lukes
James Aird 24 house painter bn Middx Islington
Mary 26 bn Scotland Orkney
Daughter , name unclear, 5 bn St Lukes
Susan 4 mths bn St Lukes
The older daughter's name (indexed Firth on family search presumably) is hard to make out. It definitely ends with "th". I wondered about Ruth, but am not wholely convinced on the first letter. It looks a bit like the "I" at the beginning of Islington in James' birthplace, but that doesn't make sense either with it ending uth or irth.
I've looked at births for 1845ish for other Airds in St Lukes Reg District, and for Reeds in St Lukes in case she was born bfore the marriage, but got no inspiration!
Birth
Oct/Nov/Dec 1850
St Luke 2 330
Susan Aird
Her birth cert would give you Mary's maiden name and thus confirm or rule out the marriage you found. Getting that should be the nest step for your friend, before following up on Mary, to make sure you get the right MAry!