I am sort of confused. Maybe it's not coming across very well to me, I am clogged up and a bit dizzy...sorry.
You sound like you're blaming her weight for the leg pain.
If so then let me tell you about me. I ran 5 miles every night, played basketball, volleyball, tennis, rode bikes all over town, even the grocery store, while in college. I was very athletic and in good shape. I was playing softball in college and in this particular game I was playing short field between first and second bases back towards the outfield a bit.
The person playing center field ended up knocking in to me during a fly ball and bent my leg sideways. Every muscle, tendon, ligament, all the stuff inside the knee, it all ripped to shreds. They used staples to flop it over and put it back together. The ends were so frayed they could not sew them together.
I did rehab for almost a year but of course my knee is not ever going to be right again. I cannot do much of anything now. I have tried walking, riding a stationary bike, treadmill, swimming, water aerobics, all of it makes my leg ache and then I have trouble walking for days.
So a knee injury is life changing. Perhaps she damaged her knee in her youth and is still paying the price much like I am.
As for the wedding, you sent the family an invitation for 2 people and 2 people are coming. What's the difference? You just don't like the son and don't want to be around him.
Sounds like you have made some decisions about this friendship and if you are certain you are done then burn the bridges and be done with her.