You won't be able to get her heart rate as that information isn't transferred via iCloud. The data stays on the iPhone.
I suppose what you could do is to have your mom's iPhone activity app share to you as a friend. It's based on activity, I think. I don't use it, but perhaps others on here do.
Based on the ads, it's like a friends leaderboard that you can compete. You'll be able to see if your mom is getting her "move" ring filled.
Here's a Link - https://support.apple.com/en-us/HT207014 on apple's website. Looks like you'd both need an apple watch. I suppose if you needed an excuse to buy a few watches, you just found it.