It's their business. No problem with it, assuming that it's an honest relationship, and not the 32 year old person trying to use the 16 year old in any way.
Sex? If the girl's 16 and the guy's 32, I think that technically counts as a crime. At least in some jurisdictions. Same if it's vice versa. Then again, I don't have much of a problem with it if both consent. It's hard to say for sure, though, whether the 16 year old can really consent. Some are mature enough at 16 to consent (with full understanding) - if so, no problem, regardless of which gender the 16 year old is. If he/she isn't mature enough to consent with full understanding, and they have sex, it's wrong, no matter who's which gender.
At the same time, if I were the parent of the 16 year old, I'd be quite unhappy, and if I were a friend of either of them, I'd probably cautiously advice against the relationship.