I really like my friend's roommate, what do I do?

Maybe do a group thing with him and his roommate and a couple of other friends (i. E dinner, club etc) just try to interact with as much as you can. Find out what your friend's schedule is like (i. E. Times he is not at home), and one day drop by and casually ask the girl where your friend is (obviously he won't be there). And then ask her if she is wants to go out for quick cup of coffee (mention your craving coffee). Most likely she will say yes (unless she is insanely busy or something). Whatever you do, do not go through him. Go directly to the girl. For all you know she could be eying you too!

Tell your friend you like her roommate and she most likely will help you with her, be your wing-girl (lol). UNLESS, this friend has a crush on YOU, in that case things will get messy. Lets hope it's not the case. Just be honest about your crush to your friend so she doesn't feel used when you hang around with her. You guys could even go out in a group setting so you can get to know her roommate better and eventually be comfortable enough with her to ask her out on a one on one date.

Don't go thru her friend, go to her directly. Trust me, your friend will get peeved if she finds out you are using her. Unless, you ask her directly for some help to get to her room mate.

Tell your friend you'd like to invite her roommate along on an outing, and when she asks why or if you like her, tell her the truth. :)

Much easier if you get her involved in the process. Even if she isn't interested in dating you, she would still be upset that you didn't share the fact you're into her roommate.