If he happens to come this way, I'll let you know right away.
Literal
By-chance he [subject-が] this-way [to-へ] came-[conditional-たら] immediately will-inform.
こちら ('this way,' 'this direction') is a directional demonstrative — politer and softer than ここ ('here'). It can mean both a physical direction and 'our side' of a conversation. 知らせる ('to inform, let know') is the causative of 知る, meaning 'to cause someone to know.'