Safely negotiate a salary for a PhD offer - Germany
One of the mods here is a professor in a german university, I think he is able to add a better answer or at least confirm or discard mine.

Some considerations:


As far as I know, many german PhD students, at least in CS, are actually employed full-time, which I assume means 100% TVL-E. In that sense, both 50% and 65% seem somewhat on the low side to me. At the very least, negotiating for 65% does not seem so unreasonable that it should offend anybody.
However, don't be too disappointed if your negotiations go nowhere. I have heard from multiple people that academic salaries are oftentimes entirely unnegotiable in Germany (though not always)
German PhD student salaries are internationally quite on the high side (also if you take costs of living into account), so even if you cannot negotiate, 50% might still be ok compared internationally. That being said, the question is always if the salary is enough for your standard of living / circumstances. If you have a partner and two kids, maybe 50% simply does not allow you to live. If you are alone and live in a dorm, maybe 50% is plenty.
In germany, some amount of teaching is usually part of the job. That the one professor told you this before is a good thing. However, there is teaching and there is teaching. I have certainly seen groups where teaching load could add up to about 75% of a PhD student's work time. This is obviously terrible (independent of money issues). The hard part is figuring out whether this is the case in your specific group. The best way is to ask one of the current PhD students.
Don't be too afraid that you may annoy your future professor. In my experience, the entire "academics frown upon money issues" is a bit of a show. The same professors who says such things at the same time have no quarrels at all negotiating their own contracts or department budgets. If your future professors gets annoyed by you mentioning that you have a number of better offers, you should likely go somewhere else anyway.


Now, for the actual negotiation, I would follow moriary's remark from the comment - simply mention (friendly and non-threatingly) that you are torn on accepting the offer, as you have counter-offers at 65%. As said, nobody should get annoyed by that. We are all adults, right?