You can say the following two in American English:

1. I'm going to my friend's house to play guitar.

2. I'm going to a friend's house to play guitar.

We don't say "the guitar" unless you are talking about a specific guitar that you have already mentioned. When you are just talking about playing any guitar in general, we just stay "to play guitar".

so, what do you need with it?. i think maybe is the way of saying it , i think is better to say :i'm going to my friend's house to play the guitar.

thanks my friend lo tomare en cuenta