Dealing with a roommate's noisy partner
Choose a calm time and name the noise, time, and effect. Ask the roommate to manage their guest through headphones, a different room, earlier departure, or advance notice. Revisit the household’s guest and quiet-hour rules together.
Do not make the issue about whether the relationship is legitimate. The question is whether the shared home remains usable for everyone.
If the visits are frequent enough to change occupancy, money, or safety, involve the lease-holder and review the agreement.