It’s customary for someone requesting a Mass be said to make the usual small donation, which in the US is generally 10 dollars.
If you can’t afford it then you tell them you can’t afford it and the priest will make some arrangement. I’ve also seen parishes in poor areas where the customary donation is listed as lower, about 5 dollars.
In USA these days, most people including those of limited means frequently spend 5 or 10 dollars on junk food, entertainment, the lottery or other non-necessities. This isn’t an unreasonable request to ask for a customary stipend. But like I said, if you truly can’t afford it then say so, donate what you can, and the priest is supposed to say the Mass anyway for your intention.
I would add that if this lady is a long-standing member of the parish, then the idea of a stipend for Mass intentions shouldn’t be something new to her. Churches have been doing it this way for decades. Every year the church will let people know the Mass intentions book is open for year 20xx and the requested donation for a Mass is “X” amount. People then go sign up at the parish office for specific dates of Masses they want, which are often anniversaries of people’s birthdays, death days, marriages, etc. The vast majority of us who have had a loved one die have gone and requested a Mass at the parish at some point and even those of us who haven’t can see the announcement regularly in the Bulletins about Mass intentions.