Yes, UNFAITHFUL is a Scrabble word!

Letter values in Scrabble US: U 1, N 1, F 4, A 1, I 1, T 1, H 4, F 4, U 1, L 1.

Is it a valid word?

UNFAITHFUL is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

not true to duty or obligation or promises
having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend
having the character of, or characteristic of, a traitor
not trustworthy