I don't know if it's the right way, but to rescue Dr. Trapper you need the fruit of the fully grown plant you have in the apartment. To get the fruit, you need to give the plant the nutrient vial you buy at Blooming and nurture the plant until it forms fruit: "kumquat-sized bulges".
The first time you harvest them. . . you will fail and end up eating the fruit, forcing you to start the process again, note that the plant will be: " . . . happy?.
After reaching the same stage again and having had sexual intercourse (anal) with her, an option "get ovoposited" will appear, accept this and read the text, at the end of the text you will have the option of spitting the fruit and keeping it or swallow it.
PS: I discovered this by accident XP
