All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The matter is as you read in the mentioned Fatwa, that it is one of the conditions for a woman to enter a restaurant that there is no forbidden mixing in it, but the mere presence of men and women under one roof is not considered forbidden mixing. Men and women were all under one roof of the mosque of the Prophet with distinction between women and men.
If a woman is committed to wearing Hijab and sits far from non-Mahram men, then there is no sin on her entering the restaurant and eating there, especially if she is with her family or female friends. It is better for a woman not to eat in such restaurants as a habit; rather, she does it sometimes if it is needed, because even if she does not mix with men, she may not be safe from some of the causes of temptation.
Allah knows best.