I think we can put elderly here instead of old

I think " The rich, old woman has her lunch in front of the expensive restautrant" should be acceptable too.

What part of the sentence implies lunch?

your answer said "lunches" - it has been decades since an American said lunches (one "eats lunch"). Also in English it implies that the lady is eating on the street (not a European street seating. You would say the woman is eating lunch out on the terrace (❤❤❤❤❤, outdoor seating area)