These days the average price for a decent dinner in Milan hovers around 15-25 euros per person for the main course, depending a lot on the area and type of restaurant. If you want a trattoria or a classic restaurant, you're looking at roughly 30-40 euros per person with appetizer, first course and a drink - nothing fancy, just good food. Slightly more upscale restaurants or ones in the center can easily hit 50-60 euros, but you're still far from Michelin-star territory.
What varies quite a bit is the location: in Navigli or near the Duomo you pay more compared to neighborhoods like Lambrate or Greco. If you're after good value for money, I'd look a bit outside the super touristy circuits. The day of the week matters too - Friday and Saturday often cost a bit more than Wednesday or Thursday.
My advice is to check a couple of places on Google Maps or review apps to see what recent customers are saying and what current prices look like. Things change fast these days, so what you saw a few months ago might already be different. For dinner with your mom, with a budget around 50-70 euros per person you're good to go - you'll be able to eat well without any stress.