No, there’s no upgrade path from Dorico Pro to Dorico Elements, because that’s a “downgrade” rather than an “upgrade”. Your friend can, however, update to Dorico Pro 5, buying the “big” upgrade here:
https://checkout.steinberg.net/91613
It might be a good idea to wait for a little while before buying the update, as users of Dorico 3.5 and earlier will have an opportunity to save on the update to Dorico 5 in the relatively near future. An email will be sent to qualifying customers, so please advise your friend that he should make sure his Dorico eLicenser license is registered in MySteinberg, and that he is signed up to receive marketing emails from us.