Is weight lifting a bad idea with healing rib fractures?

Yes: Yes, weight lifting is not a good idea with a rib fracture. It can potentially really aggravate or even potentially displace a rib fracture to weight lift.

Yes, take a rest!: Ribs are utilized during breathing and almost every movement of your body directly or indirectly. There are muscles between the ribs that are in constant motion as you breathe. In order for a bone to heal, it needs to be offloaded of movement as much as possible. The pain should be limiting you if you have rib fractures. You can allow your body to heal by avoiding weight lifting for 6 weeks.

Weight lifting : Yes initially for 6-8 weeks risk of malunion.