Now my friend wants to take divorce from her but he wants the custody of his daughter can he legally claim for her custody,
She is living an adulterous life,hence your friend can file the divorce case on the grounds of adultery by impleading the adulterer also as a second respondent, besides he can add desertion and mental agony as additional grounds for divorce in his pleadings.
He can file a child custody case under guardians and wards act on the same grounds (adultery), his plea will be considered.
second his parents gifted her jewellery at the time of wedding and it is of handsome amount keeping her behaviour in mind now they are demanding their jewellery and other precious things back from her but she is reluctant and threatened them to file a suit against them for dowry?
Legally speaking, all those gifts received by her from her husband or his relatives also belong to part of Stridhana and it becomes married women property, meaning thereby she becomes the absolute owner of such properties. However there is nothing wrong in claiming the jewels given to her for the sake of the daughter's future. Let she threaten, you can make the demand keeping the daughter's future as a shield.