Don't know if this helps, but my wife and have 4 kids and she put on about 100 lbs in every pregnancy. I am so proud of her she looks great and people are amazed that she looks the way she does. You would never know she had 4 kids. At her high point she weighed in at 262 lbs. Now after 4 kids she is 155 lbs and has very minimal stretch marks and even less around her belly. here last kid was at age 34. so her skin elasticity isn't that of a 20 year anymore. But she was always concerned with having a lot of loose skin and stretch marks around her tummy. She always oiled up her tummy through out the whole entire pregnancy and even after. She use to use so much we would have to wash the sheets every other night because they would turn yellow from all the cream that she would put on (coconut butter cream) I had to start buying it by the bulk. Then immediately after birth with all the kids she would wear a girdle religiously(for about 3-4 months or until she felt like everything went back into place, and was able to start working out again) she says this is what made the major difference. I know it doesn't sound very scientific but she swears by it and we have four kids and she has a washboard for a stomach with almost zero stretch marks. In my opinion NO stretch marks, but we all know woman they always see stuff we men never see. So that is why I say she has very little marks because my wife seems to see them. lol All in all, tell your lady the obvious things like exercise and time need to be done. My wife works out six days a week and eats very clean and now she is starting to train to compete. But this seemed to work for her. Good luck and don't get discouraged it all won't go back to place over night. After all a normal pregnancy is usually 42 weeks long of her tummy being stretched out. So you can't expect it to snap back into place in 3 months.
Good Luck!