I'm 54 3/4 and am also a grandmother raising grandchildren. I raised my daughter poorly and when she became an adult she turned to drugs and homelessness. She has 7 kids now and only one of them lives with her. I have 2 of the other 6.
I said she sounded immature and in need of someone to teach her. You said you raised your kids to pay rent and do stuff. So why isn't your son paying rent and taking care of his girlfriend and child?
You implied he was upset with her over this, it's his problem.
When it comes right down to it, which I didn't say before because "I" was trying to be nice and get you on the path of being a mother figure to her, you know, helping her learn...your son has his family living with you.
YOU should be getting money to help pay the bills. YOU should not have to be buying formula or clothes or diapers or anything for that baby you don't want to go get just because that baby is adorable and you love him. She can get a ton of that stuff free, there's no reason she shouldn't be getting WIC. The income levels for WIC are not as low as food stamps.
YOUR SON has his family living with you and it's HIS responsibility to provide for them. Not yours. You are offering them a place of refuge, a warm place with food and amenities.
You said he was mad at her for spending her money on that phone, it's HIS job to be mad at her for spending her paycheck instead of joining it with his money to pay their bills. Regardless of what she spent that money on, he still owes you for their living there.
Let him be the man of his family and let him take care of his family. Then when he can't do it because she's an teenager with a brain maturity of a 12 year old, you can step in like a friend and say "Sweetie, it sounds like you guys had a big argument, if you'd like to talk about it I'm here". Then you've opened the door to teach her about money management, about how to share parenting, about growing up. Instead of being the big bad "B" that's always yelling about money you become her best ally and she learns from you.
That's all "I" was trying to say in my post.
*********************************
So, since our answers didn't help out here's another suggestion.
Tell them you want to have a family meeting. When are they both available to sit down quietly and visit (Less dictator sounding than talk).
When you all get together open it up by saying "we've not had the chance to visit about our living arrangements yet and I'd like to get us all on the same page so things can go smoother". Or something along the lines of let's visit about our lives and come up with some positive things we can all do to be helpful and considerate to each other.
Then as the discussion moves along don't mention the phone, that's his business with her and it's between them.
The goal of this family meeting is to get them to volunteer to do stuff for you, they should suddenly realize you're doing so much you can't even sit down and rest all day....poor mom, she needs us to help her because she's been taking such good care of us. Wow, we need to step up!
Manipulate the conversation to go the way you want it to go. Keep reining in the side bars and keep it on task.
The end results should be this:
Gather and take out the trash daily...?
Sit with mom and make the menu for next week...Saturday evening, everyone.
Cook dinner 5 days per week by XXpm...?
Help with cooking dinner 3 days per week...?
Help with cooking dinner 3 days per week...?
Laundry on M, W, F....?
Laundry T, TH...?
Driving the car to transport people to work...?
Amount of money to pay rent/utilities/gasoline...?
Once it's all laid out you can put the poster board on the front of the fridge or on the hallway wall. Where everyone can see it. If they don't do their job on the day/time it's supposed to be done it does not get done. You do NOT have to do their work for them. Keep your laundry separate so you can have clean clothes to wear. Do your clothing on the weekend when it's no one's time but yours. If they don't do their laundry on their days and they don't have clean clothes to wear they wear dirty clothes. Period.
Don't rescue them. Help them grow up. Teach them both because no matter what you said about teaching your kids better and to pay their way, YOUR son isn't paying the way for HIS family or there wouldn't be any problems here.