You are in a pinch. Your advisor waited too long to tell you he couldn't pay your way, and now you can't apply for financial support through other means. (Except there is still the possibility of the co-author.)
However, you are not comfortable asserting yourself with your advisor, and requesting that he sound out the co-author; and you feel you have good reason to feel that way.
So you had the idea of sounding out the co-author yourself, and are asking whether it would be a faux pas (i.e. if it would look bad for you to talk to her about the problem).
Well, it looks a bit brash.
If I assume all the premises as you laid them out -- here's the best I can come up with for you:
Speak with Ms. Dr. Co-author in person. Explain:
Mr. Dr. Advisor and I had a bit of a misunderstanding, and I just found out he doesn't have any funding for my trip to such-and-so. I am concerned that it may be too close to the conference dates to apply for funding through such-and-so. But I wanted to ask your advice. It is a very awkward situation for me, since on my graduate stipend I just don't have the funds to finance a trip like this. Do you think I should still apply?
If Ms. Dr. Co-author listens with her heart, she will get the message.
Listen carefully to her when she responds, so you can gauge whether she got your meaning, and to get a feel whether she might be annoyed, offended, shocked, or whatever, if you were to ask her more openly.
That's the best I can come up with. Your system is somewhat over-constrained.