Disclosing a proposal
Tell the other student as many times as necessary that you just need the okay directly from the PI and then you'll be happy to forward it.

I would try to steer the PI away from okaying the forward.  It would be different if you already knew and trusted this individual.

The germ of an idea for a project can be very important.  This is your baby and I understand your wanting to protect it.

(You need not explain that to the other student.)