Hello and welcome to the group!
Since you say you have already checked the contract and there is no clause relating to series, I'd say it depends on the reason the series was dropped. Just as an example, if the early books were selling really well but the publisher went out of business, an author could have luck selling to a new publisher with proven sales, especially if s/he can get the rights back to the earlier books.
However, this is rare and I don't know of any authors who have done this. The reason being that the far more common reason for authors being dropped (mid-series or otherwise) is because of poor sales, and therefore another publisher won't take a risk on the rest of the series (or the author) if it hasn't sold well. And this applies across all genres; sales are what publishers care about.
I know of many successful authors who have switched publisher but not of any who have done it mid-series.
What I see far more frequently is authors going indie and fighting to get their rights back on earlier books in the series because they cannot get a new deal and can only effectively promote a whole series (through permafree or box sets for example) if they own the rights to every book.
Indie publishing sales have taken over traditionally published sales now and your friend may have a lot more luck going independent. If the first books in the series didn't sell well, it may be possible to buy the rights back.
If s/he doesn't have an agent, it may be worth approaching a few and getting their opinions on whether it would be possible. Agents have a finger on the trad pulse and may know of publishers seeking such work.