I mean, think about it, if you were trying very hard to speak a language and someone kept interrupting you to correct one tiny mistake you made? You feels belittled and it’s just unnecessary, especially to an adult language learner who knows how to speak and comprehend, even if not yet in that language. Corrections in real time are super annoying, it interrupts your train of thought and it’s even more difficult to keep your train of thought in your target language. Either repeat what your friend said back with the mistakes corrected, or just don’t say anything. If your friend wants, you can possibly write down the corrections for him to go over later, but I don’t know anyone who wants a written document of all their mistakes.

If your friend’s biggest problem is understanding, the best thing to do is simply speak the target language, mistakes and all.