Well here's the thing. The age of consent in WA is 16. Plus there is a 5 yr close in age provision.
So they haven't met the boyfriend. How can they object just on the basis of age? 16-19 is on the edge of acceptability. But its not enough, IMHO, to reject it without other reasons.
But also, if the boyfriend does not know of the parent's objections he really can't be prosecuted for parental interference.
So my recommendation is for the parents to insist on meeting the boyfriend before any more dates. Then decide whether to allow continued dating. If they decide to forbid it then they need to make sure the guy is aware of it and the consequences of continued contact.