While there could be problems with this relationship(you must remember that you only heard this, you need to hear it from her before you do anything) the age gap itself is not a huge problem. 1-5 years in age difference is normal. My parents are five years apart and several of their friends have similar age differences.
If you care this much about her that you talk with your friend about her, you need to talk to her as well. Do you see her around school apart from the classes and such you no longer have? Does she do gaia? can you contact her easily? You'll want to get back in touch with her by maybe saying "Hey, I havn't talked to you in FOREVER! How've you been? what's new?" find something casual to talk about, and get to know the new her(how she's changed) before you get into subjects that might get a little touchy.
maybe that's just a rumor, and it may be easier to tell one way or another when you're talking to her.