Can we assume, (rightly or wrongly), you don't really get on with them?
If you don't then it makes it harder to approach them about rectifying it. My thoughts are, if they really want that downspout to empty on to their property they could put a swept elbow in but carry the original pipe line down to the lower SVP.
Technically it would still be wrong because your svp should be high than your gutter height but it's a sort of compromise.
Your, (short length), svp is currently breaking the rules and you may find in warm weather you may be able to smell the drains because the opening is so low down.
It's very easy for people on a forum to say, "Go and demand they put it back", but in reality life is rarely that simple.
As a solution you could get someone in to extend the svp back up to its correct height with a swan neck to go around the gutter.