How long have you been married ? ;-)

Put things in perspective. Your MIL is poor, can barely afford her rent, has no internet.
She could be moving in with you anytime soon, and chain smoke in your living room.
With this in mind, the data plan is a small price to pay.
Worst case yeah, pay for a minimum internet plan and give her an old laptop and enable Wifi on the phone... Or switch her to her own plan, with limited data and no auto-billing for more data (but still pay her bill). All of that is way better than a) a divorce b) her moving in... Trust me on this!

it's peanuts vs the threat of your MOL moving with you someday. I'd pay way more than a phone bill to remove that chance. In fact, I am !

Maybe she can get some assistance with her wifi bill. Seems there's some programs for $9.95 a month.... Low Income Internet Service Providers - $10 a month

What he said, ask yourself how much the smart phone is costing you in $$$ and aggravation and balance that against what getting her internet will cost.
Just my two cents.

Yea, she can afford to chain smoke but has no money for a phone or internet.

Well that is way better than right down the street, which is exponentially better than right down the hall; glass half full and glass overflowing, respectively.

If she is just hanging out in the apartment, smoking and surfing the net, it seems like a smart phone does not even need to enter the picture.

The glass is 600 miles full

I admitted right up front that I'd done something stupid...