There are no laws that prevent your roommate from subletting without your consent. Take a look at your lease agreement, or a roommate agreement if you signed one, to see if it includes any guidelines for either of you in case you want to sublet.
If it doesn't, then your roommate may need to get your landlord's approval for the sublet—but they don't need to get yours. This is true if you are a co-tenant of the lease with them or if you're just living there and paying part of the rent.
Keep in mind that if you are a co-tenant on the lease, and the lease holds you and your roommate joint and severally liable, then you have a real financial stake in who they decide to sublet to. If they pick someone who flakes on the rent and your roommate doesn't step in, then you'll be held responsible for their share of the rent in addition to yours.
Encourage your roommate to screen the subtenant carefully, and ask for veto power over any potential subtenants.
The information provided on this website does not, and is not intended to, constitute legal advice.