Nipples are tissue that is easily expandable. Sucking on them is certainly one way as is freezing them but the results are fairly short term. Women who've nursed children however, tend to have their nipples permanently enlarged as they're repeatedly engorge and stretched through sucking. How much typically depends on how long she nurses.

As for ways to make them permanently larger by gels, creams, vacuum pumps, or "snake bite" cups, are doomed to failure except in the short term. If she's wanting the look, I have seen pasties that can be worn and emulate nipples of various sizes. But I'd suggest she just engage in a lot of foreplay with her breasts and enjoy what she has.