I lived with a common-law spouse for several years. He would like me to give him receipts for the contributions he made to our rent, going back a few years, so he can try to make a claim related to his self-employment. He believes I should have been giving him receipts all along because the lease was entirely in my name, making him some sort of sublessor. I am hesitant to issue him receipts because I believe I would then be obliged to declare rental income, and I have no idea how I would calculate expenses to offset that "income". Am I right to be concerned? As an alternative, could I just give him copies of the receipts from the landlord (which are in my name) and let him try to make his claim based on the total?
You are right to have concerns.
If the rent was split, there's no need for you to issue any type of receipts to your former spouse. Just because your name was the only one on the lease does not make you your former spouse's landlord.
However, you can make copies of the rent receipts you have and pass those along to your former spouse. CRA may request more information about these amended expense claims so I'd include a letter stating your former spouse contributed half (or whatever portion they contributed).