Sounds like a The Expanse question! (obligatory 'you have to watch that show' schpiel here).
So as you pointed out, they would possibly have trouble handling Earth's gravity. Why? Among other things, because of bone density. This is improved in 2 ways - by having plenty of calcium and related supplements, and by working out. It's not just your bones though, your muscle density also matters. If your bones get heavy and your muscular system can't handle it, it would probably cause a bunch of problems. So you'll need supplements to just improve your overall body strength. And of course, you'd have to be able to survive whatever germs you are going to be exposed to, so maybe take antibiotics too.
Here's something you can do almost nothing about - your heart and lungs. It is possible for them to simply be crushed under this newer, higher gravity. Not guaranteed, but plausible. It is possible to try and fix everything else about yourself, but still not be able to handle 1G just because your cardiovascular system is not that great.
Lucky for you, you said 0.7G. That's actually not too bad. People from your planet (except for the absolute weakest people) would (with some difficulty and intense workout regimen + diet) be able to handle Earth's gravity just fine. The extreme issues would come about if they'd grown up in much lower gravity (think < 0.3G). What would be even better? Make them train in swimming pools for a bit to ease things out. With a proper regimen like that they would probably handle it better.
Look up Scott Kelly and Mikhael Kornienko. They lived in the ISS for a year, and Scott Kelly is still not perfectly back to normal apparently. Maybe they'll give you an idea about what could happen if humans grew up in lower gravity.
Hope this helps!