If you are staying over at your friend's house and need to use the restroom, it is entirely appropriate to use the toilet. Wearing a diaper as an adult is not a typical or necessary solution for using the bathroom.
Just like in any other situation, if you feel the need to use the restroom, kindly ask your friend if you can use their bathroom facilities. It's a normal and natural human need, and your friend will likely understand.
If you're feeling uncomfortable or embarrassed about asking, remember that using the restroom is something everyone needs to do, and your friend will likely appreciate your straightforwardness. Communication is essential in any friendship, and this is just a basic part of taking care of your bodily needs while you are a guest at someone's house.