。
She'll probably be a good conversation partner for you.
Literal
She [topic-は] your good conversation-partner become-probably.
話し相手 is built compactly: 話す ('talk') + 相手 ('partner/counterpart') = 'conversation partner.' 相手 is one of the great Japanese words — it can mean partner, opponent, counterpart, the person at the other end of any interaction. 結婚相手 ('marriage partner'), 練習相手 ('sparring partner'), 相談相手 ('person to consult'). The frame XはYのZになる ('X becomes Z to Y') uses the possessive の to specify whose the new role is — here, 君のよい話し相手 means 'a good conversation partner for you specifically.' Topped with だろう, the sentence projects probability rather than certainty.