Back on the thread (yes, I read all of it
When people make mistakes with a language in a country where that language is spoken, you may assume that he will be corrected soon enough anyways.
Also, I know this urge to correct people to 'help them along'. When I meet someone who is speaking in a language that I know very well or has regular problems with coming up with the right word (in their own language or not) I always ask first 'I notice that you make a few errors, which is no biggie. But would you feel helped if I'd tip you off about them?' or 'I notice you have some problems finding the right word. Sometimes I know what you mean to say before you have found it. Would it help you if I'd fill you in there?'
Some people are fine with it. Some decline and would rather speak at their own pace. By emphasising that the choice is truly their and you are really offering to *help* and not be a know-it-all, you will likely get a positive response.
Also, when correcting someone, don't do it while they are still talking, and try to get your points across in a one-on-one conversation, so there is no embarrassment potential for the other person.