People tend to make grammatical errors, structural errors while talking in English. I have seen people who don't correct at all whoever it is, and I have also seen people who immediately correct anybody at all who makes a mistake. I don't agree with both.
Correcting a person immediately can have many drawbacks.
1. It cuts off their flow.
2. It can make them less confident about what and how they were going to say what they were saying.
3. It can be mean, sometimes (even if you don't do it with mean intentions).
4. It shows you have no idea why a person might not be great in that language.
On the other hand, if someone close to you is making an error constantly, there is no harm at all in openly talking to them and correcting them. In fact, they'll understand better and might not face such a situation again. At any point of time, the correction must be said in a manner that is educating for the other person, rather than making it sound like you are just out there to find faults.
I have had friends who have been more than happy, and in fact have voluntarily asked me to review what they speak and correct errors if I find any. And, the same applies to writing. It is alright to move with the person and their way of expression, as long as you get what they are trying to say. Remember, you might just be worse in so many other languages. Languages are many, and everybody is not required to be an expert in the same language.
You should definitely take the opportunity to teach people, and help them become better and learn more. What matters is how you do it. Wait for them to finish what they are saying and reply to their questions or statement before you correct them. It is important to acknowledge the content first, before you jump into correcting their structure. Your correction has to encourage them, not demotivate. It has to make them more confident about what they learn, and not make them lose the little confidence they have.
Most things in the world work that way, especially when you teach someone: it's not about what you teach, it's about how you do it.