Mom owns house. Lets loaf-of-a-son (adult) live there rent free. Mom discovers son is selling prescription medication out of her house. Apparently he's got several women going to multiple doctors around town getting prescriptions from all over for Soma, Vicodin, and Oxycontin.

Mom wants to let the cops in to her house. Mom has key, no rent, no rental agreement.

Do we need a search warrant? I say no. I remember the warning our Crim Pro professor gave us about 'girlfriends and boyfriends'. In a question after class a particular law student looked up at Professor Bubany looking quite nervous and asked, "So if she gets mad at me, she can just let the cops in and show them my stash? Just 'cause she's got a key?"

I think the term of art for your facts is "common authority" more than "apparent authority." It would require a lot more facts to be confident about whether you could skip the warrant, but these cases might help:

GOOD Cases for you
Williams v. State, 668 S.W.2d 692, 699 (Tex.Crim. App.1983).

Grays v. State, 905 S.W.2d 54, (Tex.App.-Amarillo, 1995, no pet.)

Howard v. State, 207 Ga.App. 125, 427 S.E.2d 96 (1993).

BAD cases
Corea v. State, 52 S.W.3d 311, 316
(Tex.App.-Houston [1 Dist.],2001, pet. ref'd)(brother, who was roomate of defendant, lacked authority to consent to search of defendant's bedroom)
State v. Peterson, 525 S.W.2d 599, 608 (Mo.App.1975) (evidence showed that the basement bedroom was "exclusively" the son's area and "no one else had a right to be there");

State v. Pinegar, 583 S.W.2d 217, 219 (Mo.App.1979) (evidence showed that the family understood a footlocker in the adult son's bedroom was his private personal footlocker).