wa'alaikumussalam warahmatullahi wabarakatuh
Indeed it is a problem not only with one person or two. Many and many people of this Ummah have this thinking.
Try to advice her like this if other ways do not work.
Allah (subuhaanahu wata'aala) made us in a very weak situation that we do not know anything to do other than crying hard. He gave us Rizq, People who will take care of us (Parents), a brain from which we can think, eyes to see, ears to hear, feet to walk, hands to work, mouth to talk etc. After giving all those trillions and trillions and trillions of Ni'mahs which we cannot count even if we try, as Allah says in Quran,
"And He gives you of all that you ask Him; and if you count Allah's favors, you will not be able to number them; most surely man is very unjust, very ungrateful." [14.34], He showed us the correct way from the two, by sending books and Prophets (may Allah have mercy on them all) to teach the correct path.
Life of this world is nothing but a very short period of time. The person who struggles here will be the person who will be delighted at the end. Jannah is covered with hardships and difficulties. If we want to enter it, we have to face them. Jahannam is covered with easiness and laziness. Entering it, will be really easy.
What we always remember throughout our entire life is this Hadith of Prophet (sallallahu 'alaihi wasallam).
"Whoever seeks Allah’s pleasure at the cost of men’s displeasure, will win the pleasure of both; as Allah will cause men to be pleased with him. Whoever seeks to please men at the cost of Allah’s displeasure, will win the displeasure of both; as Allah will cause men to be displeased with him." Bukhari and Muslim.
As per leaving the Hijab because people's words or thinking, I would recommend to study this Hadith before doing it.
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: Two are the types amongst the denizens of Hell, the one possessing whips like the tail of an ox and they flog people with their help. (The second one) the women who would be naked in spite of their being dressed, who are seduced (to wrong paths) and seduce others with their hair high like humps. These women would not get into Paradise and they would not perceive the odour of Paradise, although its frag- rance can be perceived from such and such distance (from great distance). - Sahih al-Muslim Book 40, Number 6840.
Hope this helps you my dear sister. Keep utmost trust in Allah in calling her to the truth.
wassalam 'alaikum warahmatullahi wabarakatuh