He is living in Germany – should he fast with the European Islamic Society or with Saudi?

If the
European Islamic Society depends on sighting the moon, then it is better for
you to follow them with regard to starting and ending the fast. 

There is an
ancient and well known difference of scholarly opinion concerning this
matter – should each land have its own sighting or should everyone follow
those who see the moon even if the sightings differ from one country to
another? This difference of opinion is permissible and is based on ijtihaad,
and there is no sin on anyone if he follows one of these two opinions,
according to the evidence that becomes clear to him. Perhaps the more
correct view is that the difference in sightings is relevant in this issue.
We have explained the evidence for this view in the answer to question no.
50487. 

Among the
evidence that supports that is the report narrated by Muslim (1819) from
Kurayb, that Umm al-Fadl bint al-Haarith sent him to Mu’aawiyah in Syria. He
said: I came to Syria and did her errand, then the new moon of Ramadaan came
when I was in Syria, and I saw the new moon on Thursday night. Then I came
to Madeenah at the end of the month and ‘Abd-Allaah ibn ‘Abbaas (may Allaah
be pleased with him) asked me about the new moon. He said: “When did you see
the new moon?” I said: “We saw it on Thursday night.” He said: “Did you see
it?” I said: “Yes, and the people saw it, and they fasted, and Mu’aawiyah
fasted.” He said: “But we saw it on Friday night, and we will carry on
fasting until we complete thirty days or we see it.” I said: “Is not the
sighting and fasting of Mu’aawiyah sufficient for you?” He said: “No; that
is what the Messenger of Allaah (peace and blessings of Allaah be upon
him) enjoined upon us.” 

This
indicates that when lands are far apart, each land has its own sighting, and
it is not obligatory for everyone to fast when one land sees the moon. 

And Allaah
knows best.