Does she see this as a problem? Because she's going to have to be motivated, in order to fix it. If she acknowledges it is an issue, then she should see her ob-gyn for an opinion. This is relatively common for women who have had a lot of kids, I would not take it as evidence she is having an affair with a guy who is really big (it would take someone as big as an infant being born, to make a vagina lose its normal shape, and men aren't built that way.) She can do pelvic-floor exercises, and she can do other things, but there are also surgeries that can be performed. The main thing is for her to see a doctor, just to be sure nothing more serious is wrong. In the meantime, hands and mouth can be pretty interesting and useful tools in sex.
Hi Puff, First just how many children has she given birth to, its a big factor for vaginas, so what can she do , well pelvic floor exercises will help and she needs to have a glass dildo, because of its weight its hard to hold in, so this gives a muscles and chance to grip it to hold it in, so this improves her muscles, and thus takes up the slack your on about, and give you a better ride.
Any help.
Good Luck