Nod your head slightly after each set of words - whether they are right or wrong. The speaker will take this as a cue to continue.
The speaker may ask, "Is that right?" in the middle of the sentence. If it is, nod your head and make a "go ahead" gesture with your hand. If it's not, just make the "go ahead" gesture and nod very very slightly. The speaker doesn't know it, but this is the worst possible time to provide verbal feedback -- especially negative feedback. The goal is to have the speaker finish the sentence.
If the sentence was right, tell the speaker it was right. If it was wrong, don't say it was wrong. In both cases, repeat the sentence (or its idea) to model it in correct English. This provides reinforcement for correct sentences and correction for incorrect sentences. It also lets the speaker know that you understood the meaning of the sentence. Examples:
Speaker: My friend owns a restaurant.
You: Yes, that's right. Your friend owns a restaurant.Speaker: He buy book yesterday.
You: He bought a book yesterday.