Divorce

I'm advising a young man in a divorce case, however there's a problem I
can't answer: he's Turkish, but has Danish citizenship,as has his child, his
wife however is Turkish citizen. In Danish law it is possible to continue
joint custody after divorce, but if his wife, being guardian, brings the child
to Turkey and there applies for sole custody, will he be able to claim the
child back on grounds on 1)the Danish rule that he also is guardian and
has custody, 2)the child being Danish citizen? or will she be given the
custody on grounds of it being better for a Turkish child (being child of a
Turkish mother it has the right to Turkish citizenship) to grow up in
Turkey?

Yes Turkish court can decree to give the custody to Turkish mother on
the grounds that you've mentioned in your message.