After her birthday, Diana is Primed and Pumped to do something about both the paperwork to apply to her major (unless she already did that) and, more importantly, do something about the fact that she likes someone and they seem to like her and SEIZE THE DAY and such.

Except a day or two after she gets sick she ends up incredibly sick. Not Quite hospital sick, but enough to be bedridden or just super out of sorts and just enough to drag herself to class. (optionally: she gets the different students, including Palmer, to give her info on stuff she missed out on.)

All the LIs check in on her to some extent, probably even regardless of route but Madi lives with her, so.

In regards to Madison route lock specifically, there's a couple options: either Madison already knows she's sick, or Madison... somehow doesn't realize she's sick when she tries to do this. Either way.

It turns out that the Valentine's Day gacha is still running on the game they mutually play, and Madison decides to both keep an eye on her, (likely help with any homework/exam studying), and play Di's account for her. Because it must be pretty dire if you can't play mobile games.

Anyway, Madison's goal: roll Diana's favorite character, who is a limited 5*/SSR on her account, using her own cash/card, get it in a few pulls, and do some cheesy line about how Diana's her lucky charm and just. Generally fuss over her and show that she cares and maybe even work up the nerve to confess.

Diana however is high out of her mind and not in the weed way. Madison has slow luck pulling her character of choice, and by the time she does it, Diana's already pretty much conked out. Madison takes the hint, the maidenhearted girl.

Alternatively: Madison got Robin or someone else to help her make the soup. She put in something questionable. Madison is Trying.