Though dilettante might sound like a nice French word, don't use it on your friend who thinks he can play the guitar after several short lessons. A dilettante is an amateur, often one who pretends to be very knowledgeable.
The meaning of dilettante has changed since it was borrowed from the Italian in the mid 1700s. Originally, it meant "lover of the arts," but began to take on a negative slant as the idea of doing something as a professional took hold strongly during the 18th century. A dilettante was a mere lover of art as opposed to one who did it professionally. Today, the word implies you're pretending to be more of an artist than you're interested in or capable of being, so if you call your friend who likes to paint a dilettante, it's like you're calling him or her a poser.