If so, aren't they going to use XRP coin? Please confirm, I am a bit confused in this.
This question of yours is the same question that cointelegraph raised to Ripple, whether this partnership to facilitate instant remittance would use XRP. Apparently, the company has not yet issued an official answer or statement pertaining to that.
Bitcoin is never an option to deals such as this. It might end up a one-man deal, or not a deal at all. There is no one who represents Bitcoin, who would sign any legal contract on behalf of Bitcoin, because Bitcoin is not owned by any single individual nor existing as a company.
Also, if I am not mistaken, Bitcoin is illegal in Bangladesh.
As to its legality, are we seeing any contract or official partnership deal or agreement document? I think there is none publicly available. The question of legality is hard to answer because we have very limited knowledge as to the partnership. In fact, it was only accidentally discovered after a random twitter user "pointed out the Bank Asia logo in an official Ripple document." Outside of that tiny piece of seemingly insignificant information, we have nothing to bring us into a conclusion that the two parties indeed signed any partnership deal.