They live in Holland – with whom should they start fasting?

Firstly: 

According to sharee’ah, the onset of Ramadaan cannot be
proven except by sighting the new moon, because the Prophet (peace and
blessings of Allaah be upon him) said: “Fast when you see it and break the
fast when you see it.” Narrated by al-Bukhaari, 1909; Muslim, 1081. There is
no room for the use of astronomical calculations in proving the onset of the
month. 

Undoubtedly the moon sighting varies from one country to
another – especially those that are far away from one another. The dispute
is not about differences in moon sighting, for no one disputes that. Rather
the dispute is whether the differences in moon sighting have any effect on
proving that Ramadaan has begun from one country to another. 

Secondly: 

If Muslims who live in non-Muslim countries have an Islamic
organization or council to whom they can refer and depend on their moon
sighting to confirm that the month has begun and ended, then the Standing
Committee has issued a fatwa stating that this council comes under the same
ruling as an Islamic government for those Muslims, so they must follow them
with regard to the beginning and end of the month. 

The details of this are given in the answer to question no.
1248. 

But if they do not have an Islamic organization, there is no
sin on them if they follow a country that they trust, which follows the
moonsighting according to sharee’ah, not astronomical calculations, so they
start and end the fast with that country. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked
about a person who was staying in Spain during Ramadaan and fasted according
to the Land of the Two Holy Sanctuaries. He replied: 

What you have mentioned
about your fasting with us and breaking the fast with us, because you are
staying in Spain during Ramadaan, there is nothing wrong with that and there
is no sin on you. The Prophet (peace and blessings of Allaah be upon
him) said: “Fast when you see it (the new moon) and stop fasting when you
see it, and if it is cloudy and you cannot see it, then complete the number
of days as thirty.” This is general in meaning and applies to all of the
ummah. The Land of the Two Holy Sanctuaries is most deserving to be followed
because they strive to rule according to sharee’ah – may Allaah increase
them in guidance – and because you are staying in a country which is not
ruled by Islam and its people do not pay any attention to Islamic rulings.
End quote. 

Majmoo’ Fataawa Ibn Baaz,
15/105 

For more information please see the answers to questions no.
1226,
12660 and
1602. 

And Allaah knows best.