rented a room to a guy and he bought a roommate

If the other room mate is residing with the original tenant then the landlord can question about it and ask the tenant to vacate for breaching the agreement conditions.

Failing which the landlord can take legal action on this

It is generally considered a violation of the terms of a rental agreement if a tenant brings in an additional roommate without the landlord's consent. As a landlord, you have the right to determine who lives in your property, and it's important to communicate your expectations clearly in the rental agreement. If you did not explicitly allow the tenant to have roommates in the rental agreement, you may have grounds to take action. Here are some steps you can consider: 1. Review the rental agreement: Check the terms of the rental agreement to see if it explicitly prohibits subletting or having roommates without permission. 2. Communicate with the tenant: Reach out to the tenant and express your concerns about the additional roommate. Ask them to remove the roommate or obtain your consent in writing. 3. Send a formal notice: If the tenant fails to comply, send a formal notice stating that they are in violation of the rental agreement and request that they correct the situation. 4. Seek legal advice: Consult with a lawyer specializing in landlord-tenant law to explore your options for resolving the situation. 5. Consider eviction: If the tenant continues to violate the terms of the rental agreement, you may need to consider eviction proceedings. Remember to always follow your local laws and regulations regarding tenant rights and landlord responsibilities.