Competent surgeons don't ever promise that there's zero risk of long-term negative side-effects -- there's always some risk, even if it's miniscule. But I was told that even in rare cases where there is post-op diplopia that it almost always goes away over time (and if it doesn't, reoperation is always a possibility.)
it is extremely unsettling, though, to have your vision change in any way, even if it's temporary -- so your girlfriend should be aware of that and be ready not to freak out if things seem different or a little confusing at first, as they'll seem more "normal" as her brain adjusts to the new data it's receiving.
Her surgeon should be able to address all of these concerns, though, and if not, i'd go with another one. Vision is so personal and sensitive that you need someone who can communicate well about all of these issues.
I consider myself very conservative about these decisions, but i'm very glad I went ahead with the surgery after all.