How do I fix the frame that broke on my couch?

Hi Mona, adding support beneath the frame would be my idea. Create new legs out of 2x4s to put underneath where the couch broke and screw it to the frame.

If you can get into the broken piece, pull it straight and use Liquid Nails (come in a caulking tube) to glue it in place. You may need to wrap some wire around the support until it dries. Then staple the bottom cover back on and add additional support under that spot. I used a cinder block under mine for 10 years and nobody knew it was there except me!

Liquid nails won't hold under use. Put the pieces of broken frame together and screw a board across the break.

Turn it over if possible and carefully remove the fabric on the bottom. You can then see up inside and locate the break. Screw and reinforce if possible - it's worth taking your time and making sure the problem is fixed before reupholstering the bottom

Screw and use wood glue that should hold I use gorilla wood glue then screw it together

Rather than expending this effort yourself, why doesn't your adult son purchase you a new sofa?