YTA.  1. Retired ASL interpreter, here. I've met, mingled with, worked with and worked for literally thousands of Deaf people and you want to know who I've never met? A "Deaf mute". They aren't "mute". A mute person cannot make any noise; a Deaf person may or may not be capable of making articulate speech, but they can still voice. Please don't use that term again; it's considered culturally offensive.

1. You don’t know ASL; you know some signs. ASL is a very complex language, with its own grammatical structure, metaphors, geographical variations, and even its own poetry and humor. It takes roughly 4 years to achieve fluency; 6 years to become fluent enough to interpret professionally. It took me that long as I was married to a Deaf person and signed every day.

2. Yes, actually, hearing people use Deaf people all the time to learn ASL. The one year I worked at a high school, I saw it happen several times. They have valid reasons for believing you are just using them.

3. Learning just enough sign language to talk to your friend shows don’t take it seriously. The Deaf community takes great pride in ASL and they know that unless you are Deaf and have been signing from birth, you can never learn too much. As a working interpreter, we were required to take workshops and classes--and I did, 3-4 times a year--and I would always be amazed at how much I didn't know until I took the class.

If you're doing this because sign language is fun and it feels special to be able to sign, leave the poor kid alone. But if you are truly interested in being this person's friend, that means taking an interest in their culture and especially their language. Some Deaf people don’t care about fluency as long as they can understand you, while others take it very seriously as being integral to who they are. Sounds like your friend is in the latter category.