you sound vindictive and I’m not sure what makes people act in this way. It’s quite callous and says more about you than her.
Firstly, do you expect someone that’s off sick to never leave the house? Because at a bare minimum, they would need to leave the house to do certain things like attend medical appointments, collect medication and grocery shop. These are basic things focused on survival. I’m sure you don’t expect her not to eat during this time.
You’re acting like you saw her at the pub dancing on the tables/having loads of fun. You saw her shopping at Tesco in reality? Like get some perspective here - she wasn’t doing anything in breach of work sickness policies. You’re randomly jumping to gross misconduct, but that’s not the case. You would look crazy for reporting her for this.
Secondly, if she has a toddler - part of what you saw might be her putting a brave face on for her toddler. Are you expecting her to not smile or laugh at the toddler because she’s off sick from her NHS job? Are you expecting her to not take the child out? You don’t know what her childcare options are? It’s not the case that she can switch off being a mother because she’s off sick.
Finally, an actual medical professional assessed her and gave her the fit note. So your visual assessment of her pain doesn’t mean anything, because you are not her physician. It could be the case that the doctor feels her pain is caused by work for example, so authorised time off for the sake of improving her condition. Or it could be that she’s taking medication which is working, but she needs to rest for 4 weeks alongside that.