Not so quick and simple. :-) "Ich denke, dass es besser für mich ist." is correct standard German, so I'm not sure what your friend means. In standard German, you can't leave out "ist". Of course, while texting and in colloquial speech, you sometimes don't use complete sentences or avoid unnecessary subclause constructions, so I'd have said something like "Das ist wohl besser (so)". In doubt, ask your friend what he means. :-)

I have learnt this rule and I think you have written correct, when I was in Germany germans sometimes don't spoke right from grammatik view, but if you say without verb people would understand you