I need to vent!

[Ode to a Grasshopper]

the day he decided to "tag along on our trip" (A.K.A take over our trip) i said to him like 4 or 5 times "i dont really want to stay with [Friend A]'s Grandma, cant we just stay at a hotel and each time he's response was "nah, we'll just stay with her, it's better that way"

Here's where you're going wrong: you don't ask if you can stick to your original plans, you say what your plans are and if friend B doesn't like it he can do something else. It's your trip.

Friend B claims the reason he doesnt want to stay at a hotel is to save money (which is understandable) but i'm pretty sure his reason is actualy that he wants to see Friend A's Cousin again (since he "likes" her) and he is more likely to do that if he stays with Friend A's family (grandma)
funny thing is Friend A doesnt seem to get a say in Friend B's plan either even though it's his grandma

If that's not diplomatic enough for you, "Grandma says there's not enough room for three people in her (presumably small) Japanese house, and I'm not comfortable imposing on her anyway. We'll catch up with my cousin at a bar somewhere."