Yes, even you think the stuff is not hard to do, but it may not be true for others.
Therefore, this is a good chance for you to shine. Yes, do it! But, before you do it, you need get your manager approve and be sure he/she is happy, so you will get good performance review later.
Email to B:
Thanks for the email. Yes, sure I will be happy to help.
As you know, Mr. Nice Boss is my manager, I have to get his/her approve first. Let me report to him and then I will more than happy afer his approve.
Email to your manager:
Boss,
I appreciate your gave my contact info to B, thanks!
I just get this email from B, I would like to know how do you think about her requirement, will you approve me to help them on this project?
Yes, I have things to do right now, however, it is OK for me to arrange time and help them if you do approve.
CC: B's email to boss.