When a marriage contract is executed between a man and a woman, with all conditions fulfilled – the presence of a guardian, offer and acceptance, the presence of witnesses – the man and woman become legally husband and wife.
It is lawful thereafter for them to be alone in one place and to have sexual intercourse.
However, the prevailing customs where people live should be respected as long as they do not violate Islamic Law. It is known that in many societies that it is not deemed proper for the marriage to be consummated until after the wedding party is held.
Therefore, I advise her to convince her husband not to consummate the marriage until the wedding party is held and to follow the prevailing custom. This is in order to protect her and save her from what people might say.
Even though the man and the woman have the legal right to have sexual intercourse, they can hold off doing so until the wedding party is held, in order to respect the prevailing customs in the area and to protect themselves from any possible scandalous speech that might affect them or even the reputation of the woman’s family.
Her parents’ opinion is correct. And she has to accept their advice.
The best solution for them is to have the wedding party as soon as possible.