Boyfriend's crazy landlady is trying to kick him out because we had sex :(

I live in California. My boyfriend signed a lease with a new landlady on July 13th and moved in on July 15th. He's renting a bedroom from a three-bedroom house. The second bedroom is occupied by the landlady, and the third bedroom is occupied by another tenant who's been living there for four years.

Today is July 21st. This afternoon I was over at his place and no one was home, so we had sex in his room. Afterward, I went to clean up in his shower and he stayed in his room to tidy up. While I was in the shower, his landlady came upstairs saying she heard noises and asked what was going on. His door was already a bit ajar, because I didn't shut it all the way on my way to the bathroom (I thought no one was home). She pushed the door open to see my naked boyfriend and started freaking out, asking him who was in the shower (she could hear that it was on), telling him it was not okay for him to have a guest over, especially if the guests use the utilities, and stormed downstairs after yelling all of this loudly enough for me to hear over the running shower. I ran back into his room, scared and confused and dripping wet (not one of my finest moments). He was putting his clothes on as I came back in, and went downstairs right away.

They talked downstairs for about ten minutes while I hid in his room, and when he came back upstairs he told me he had a to find a new place to live. Apparently, she told him he was not allowed to have guests over (1. He didn't see that on the contract when he signed it, but he's going to get a copy of it to make sure; 2. The other tenant brings one of her friends over frequently). She told him he has a week to find a new place to live, and then she's going to kick him out because "she can't trust him anymore." She also said she's not going to give his security deposit back.

Is she allowed to kick him out for this? He's been living here for less than a week, and he paid for an entire month of rent. Obviously he doesn't want to live here anymore regardless of the outcome, but it would be preferable if he could stay through his first month or if she could give him his money back. I especially don't understand how it's legal for her to keep his security deposit. I don't know the exact language on the contract regarding eviction, but she has to let him stay at least until the end of the month, right?

Thanks for reading, and any help is appreciated.

Edit: Wall of text. Sorry, new to this :(