Just couldn't pass up on this question. First of all, your teacher sucks big time when he/she said that you can't combine tenses within a sentence You can certainly combine tenses to express various times in a sentence.

The correct way to say what you're trying to say is as follows,

A friend of mine, who used to be member of various support groups, is always very kind to people around him, and a great listener as well.

他以前因為參加過服務性社團, this was a past fact, so you can definitely use "used to". "Used to", it's used to describe something(action) that "was" true in the past, but not anymore (present & future).

所以他很對人很好,也很會聽別人的心事(過去對人很好,現在也一樣), this is a fact. Meaning that in the past he was kind to people around him, and a great listener. In the present, he still is, and he will be that in the future as well. So it's a "repeated action", so the correct tense to use is "Simple Present Tense". Now, if you were to use "Simple Past" here as your "teacher" has indicated . Then all you're saying is that your friend "was kind and a great listener in the past", but "not anymore"... Meaning that now, he/she isn't kind and a great listener, and certainly won't be kind and a great listener in the future either... I don't think this is the idea that you want to express Therefore, your initial inklings were right, and your teacher is the one who screwed up

I hope this little explanation helps. Just my 0.02cents.