No, they're not synonyms, though they're often used interchangeably.
Friendly means welcoming, receptive, warm, favourably disposed to other people, someone who makes an effort to make others feel at ease.
Likeable means that a person is pleasant and has qualities that make them appealing to other people, and easy to like.
A person could theoretically be likeable (because he's charismatic or attractive in some indefinable way) but not necessarily friendly.
Similarly, a person could be very friendly to other people, but have something antipathetic about his personality that means that he fails to generate positive feelings in other people.